I'm all about symmetry, I gotta balance the sonic energy on one side with something on the other. I usually record a separate rhythm guitar part doing something different from the first rhythm guitar part. Like an acoustic on one side, an electric on the other, then the lead electric down the middle.
Or i might use a keyboard to complement the one rhythm guitar.
But maybe just put the rhythm guitar on one side, the lead/fill guitar on the other and not pan them too far from center. Then the lead/fill would have to be more busy than an occasional fill.
Funny, I'm going to be facing the same decision soon with my own band. We are about ready to record a CD of 12 songs. Two guitar players, each does rhythm and lead, whenever and where ever they want. I think in this case, it will be easy because both will be playing most of the time and will balance each other out. My question will be whether to pan the solo's to the center or leave them on their respective side.
Like I said, I'm all about symmetry.
Another idea. Sometimes I like to use a ping pong delay on the rhythm guitar. The main signal would be centered but it does a nice delay to each side. I set the delay time to about 160-240ms, depending on tempo, and set the feedback level to one or two repeats. It makes the rhythm guitar fill out in the stereo field without getting the Haas effect.