Stunt Car Racer (with turbo-mode, lets say with 030/50) is a perfect example of linkup game.
And Super Skidmarks, we've played that a lot last years on meetings with simultaneous 8 players! You can split display between two Amigas and have 4 players on each computer.
In old times I used to like Knights of the Sky, Lotus and Armour Geddon as linkup game too.
Oh and if you have beefed up Amiga capable to hires gfx fast enough, Napalm is just great in two player mode!
