Dissimilar rigs sounds great. In fact, going a little in the face of what I initially replied with, different rigs per side does
make the mix sound much bigger than if it's the same rig, or very similar. What I meant was; don't get too bogged down in blending amps for the same guitar take, or on the same side (i.e. for the same guitarist's part) in the interest of making the guitar tone in general sound bigger/thicker. It should only be done in the pursuit of tone, and only then if it's really necessary.
Anyway back to the point... If you're worried about the amps being vastly different beasts, putting them through the same cab will go a very long way to bring more similarity between them, so the differences aren't so vast.
If you have an identical rig left and right, it sounds totally and utterly fine, but it tends to make the mix on the whole quite bland and spaceless. You get this illusion where it sounds quite flat, as the separation isn't all that great. But two different rigs - lots of space. It creates this faux void in the stereo spectrum and sounds a lot wider, cleaner, and everything feels like it has more of its own pocket.
So don't be worried about lack of similarity. It's obviously stupid if you've got like a triple rectifier through a mesa OS 4x12 one side, and a Vox AC30 the other
But two high gain heads, through similar cabs, will sound absolutely great.
If you'd like an example of how it works, listen to Slipknot Vol 3. There are quite a few tracks where one guitar drops out (intros etc) and you can REALLY hear the difference between the rigs the two guys were using. Dark and scooped vs bright and spongy. But bring them together and it sounds huge (plus, when they're brought together, the differences disappear as you're hearing the guitar mix as one cohesive sound).