Another tip taken from Frank Filipetti’s GearFest seminar on mixing using the Universal Audio UAD-2 card (NYC mixer/producer/engineer/studio owner Fab Dupont espouses a similar approach in his mixing seminars):
Set up three stereo reverbs (Frank uses three UAD-2 plates), one with a short room preset, another with a medium-long ambience, and the third with a long reverb. By selectively routing signals through the different reverbs, you can place all the tracks in a room together, as well as add in extra ambience where needed or as an effect.