Working the Same Case

The first team is investigating a man found shot dead downtown. In the B-plot, the second team is examining a woman found strangled dead in her bathroom. Turns out they both had the same hairdresser. Could it be that both teams are Working the Same Case?

This is a useful technique to add a minor twist to help shows with two concurrent cases feel a little less formulaic. Used at least once a season in CSI.

A variant is for the senior officer, fed up of the Cowboy Cop's attitude, to tell him he won't be involved in the big murder case, and will be investigating a break-in at a laundromat the night before. The Cowboy Cop will soon realise, possibly in a Eureka Moment, that it's the same case. He will be careful not to let anyone else in on this, assuring his superiors (truthfully) that he's still following up that laundromat robbery.

Examples of Working the Same Case include:

Comic Books

In The Simping Detective, Jack discovers that he and Galen Demarco are working the same case. Jack is investigating the deaths of several mob underbosses, while Galen is looking into the disappearances of alien prostitutes. Galen wants Jack to back off, until Dredd suggests they work together. Turns out, Bob, the bartender at The High Dive, is drugging the girls with a chemical which reacts violently with semen, causing massive explosions.

Literature

Older Than Television: This trope has been around a while. In almost every Hardy Boys book, the boys and their Police Chief father were Working the Same Case.
Harry Dresden in The Dresden Files will often end up working separate cases that turn out to be related, though in later books his problems are just as often unrelated.
In the Robin Cook novel Blindsight, Laurie gets into repeated arguments with her police friend about which is more important: a string of cocaine overdoses among previously upstanding rich kids, and a series of gangland murders. It turns out that a recently blinded mob boss is a bit impatient for his cornea transplant and is working on both ends of the problem. In order to make sure that the organ donors were in an acceptable condition, he had them die of a forcible cocaine overdose, followed by being stuffed in the refrigerator until the police arrived. As for the waiting line of patients, it didn't particularly matter how they died.
Thud! - whilst most of the Watch are trying to find out who killed a dwarf rabble-rouser, Fred and Nobby decide to investigate an art theft, with the specific intent of staying very far away from rioting dwarfs. It doesn't work out that way.
Happens occasionally to Stephanie Plum and Joe Morelli. Tends to lead to her winding up in dangerous situations and/or blowing his case because he never tells her things she needs to know.
A common occurrence in the Retrieval Artist series, when Miles Flint, the retrieval artist working outside the law, is investigating the same affair that the Armstrong lunar police force has been tasked with, for his own purposes.
Shows up in Elizabeth Honey's Remote Man. Kate is outraged by the disappearance of a rare native python and begins investigating the man she believes responsible, an American tourist nicknamed the Cowboy. She attempts to get her cousin Ned, currently staying in Massachusetts, but he and his friend are sidetracked when the bear they had been searching for in the forest is killed and her cubs are stolen. About halfway through the book, we learn that the Cowboy is responsible for the bears and several other poaching jobs worldwide.
Anita Blake: Back when the books still had plots, whoever tried to hire Anita to raise a zombie in the first chapter would somehow be involved in the main mystery of the book.

Live Action TV

The Pilot for Monk included Monk being called in on two unrelated cases, in fact by two different police divisions, only to later discover that the same man committed both murders.
Veronica Mars has a habit of cleverly combining Veronica and her dad's cases, sometimes pulling dismissed plot threads into one cohesive reveal of the season's Big Mystery.
Law and Order: Criminal Intent two-parter, In the Wee Small Hours. Both teams involved. And a fake Nancy Grace. Heavenly.
- In the L&O:CI episoed "Purgatory", Eames gets a temporary partner to investigate the shooting of a dealer and two tourists. Meanwhile, a suspended Goren is working undercover as an enforcer for the big dealer suspected of ordering the hit.
In L.A. Confidential it turns out that all three of the good cops (or anyway, the not so bad cops) are working different angles of the same case.
CSI had an interesting subversion in that two twins who were Separated at Birth were killed within an hour of each other. The causes of their death have nothing to do with one another and in fact, it seems that there are two separate murder victims, both of whom are the result of very separate lifestyles. Then its revealed that the twins both used the same dry cleaner. Turns out that the one murder suspect was found out by one of the twins to be photoshopping war photos and he decided to meet her when she picked up the dry cleaning, killing the first twin (the one who wasn't his target). He then went to the real targets house to eliminate the proof of his forgery only to be walked in on the second twin (the intended target). In his justified panic, he kills her as well. Gil calls it as both, seeing as how one case was with a motive and the other was a case of wrong time wrong place, but they were both killed by one man.
- Of course, the show also plays the trope straight in a great number of episodes.
In Kiss Kiss Bang Bang Harry Lockhart realises that Perry's surveillance job (and the murder that occurred during it) and the disappearance of his friend's sister are connected because the same plot device was regularly used in a pulp fiction series that is significant to several of the characters involved.
Despite having two detective teams, Law and Order:SVU avoids this, as generally one team works with the direct suspects and the other handles either the evidence or tries to get info from more obscure places.
In the Castle episode "Double Down", Castle and Beckett have a bet with Those Two Guys about who can solve their murder first. Each team finds a suspect, but they both have alibis for their respective murder. The teams call off the bet and join forces when forensic evidence is found linking the victims. Ultimately it turns out to be a Strangers on a Train Plot Murder.
Easily half the episodes of The Good Guys aired to date rely heavily on this trope. Dan and Jack are intentionally sequestered in the career dead-end of small property crimes, yet as Jack attempts to do his job properly and Dan whines that they ought to be "out there bustin' punks," they inevitably stumble across a much bigger crime in progress, often the ongoing major case their lieutenant specifically barred them from pursuing.
- When Dan is framed for a kidnapping, Jack ignores the theft case he is assigned to and instead pursues the kidnapping case against orders. In the end it is discovered that the kidnappers committed the theft as well so Jack is in the clear.