Reverb diffusion can usually be narrowed within the reverb plugin itself. I've done things like using three separate reverb busses: one stereophonic and normally spread for the lead vocal, and two narrowed and panned L/R for the backing vocals and hard-panned instruments (usually panned to the opposite reverb bus as their mix pan). If your reverb plugin can't do it, you can use Channel Tools to steer them where you want, as Steve suggests.
It's actually a neat effect, but a) not very natural-sounding and b) too much of a hassle to use every day. Maybe I'm too old-school, but I think of electronic reverb as an emulation of a real-world phenomenon. And in the real world, natural reverberation isn't neatly separated into left and right - that smeary muddiness is the reason for reverb in the first place!
Another easy trick for getting better separation: use a delay or a ducking compressor in front of the reverb. This will let you use fatter, fuller-spectrum reverbs without them muddying the mix. As a corollary to that technique, try using a ping-pong delay or tempo-synced auto-panner in front of the reverb. No, it surely does not enhance separation or clarity, but it sounds cool.