It's Kirk vs Picard...over a woman!
(This does not count actors meeting their later counterparts like Buster Crabbe on Buck Rogers in the 25th Century, Christopher Reeve and Helen Slater on Smallville, or Adam West on Batman the Animated Series!)
In this case, Chris Pine (Captain James T Kirk in Star Trek ) and Tom Hardy (Captain Jean-Luc Picard clone Shinzon in Star Trek: Nemesis ), making it an alternate-universe Kirk vs a clone Picard in the new film This Means War.
And, while the flick is being promoted as a romantic comedy, it's also an action-adventure/spy pic!
Here are some other "sorta" movie/tv team-ups...
The Lone Ranger meets Commando Cody!
Clayton Moore (The Lone Ranger on The Lone Ranger tv series) played the lead villainous henchman in Radar Men from the Moon!
Batman meets Superman
Robert Lowry (Batman in the Batman & Robin serial) appearing as a government agent on Adventures of Superman!
The Shadow meets The Green Hornet
Victor Jory (The Shadow in The Shadow serial) playing a villain on The Green Hornet !
Captain Midnight meets Captain Kirk
Richard Webb (Captain Midnight on the Captain Midnight tv series) as a psycho Starfleet officer on Star Trek!
Doc Savage / Tarzan meets Superboy
Ron Ely (Doc Savage in Doc Savage: the Man of Bronze & Tarzan on Tarzan [1966-69]) portraying the retired Golden Age Superman on Adventures of Superboy!
James Bond meets Superboy
George Lazenby (James Bond in On Her Majesty's Secret Service) as Jor-El in Adventures of Superboy.
The Batman meets The Shadow
Adam West (Batman on Batman ) met Alec Baldwin (The Shadow in The Shadow ) on 30 Rock!
For the first time, the meeting of two actors who played heroes, and neither was a heroic character!