While not the famous amp-killers that the Kappa 8 and Kappa 9 speakers are, like many vintage Infinity speakers, the RS-IIIb's are not the easiest load to drive, and do have impedance dips in the bass region that approach 2 ohms.
Infinity rates the nominal (aka "average") impedance of both the IIIb's and 5b's as 4 to 8 ohms. If you have the speakers wired in series, you're probably okay. But if you are running them in parallel (and most amps with A and B speaker connections do), the amp sees half of the rated impedance, or a nominal 2 to 4 ohms. If you were playing, say, some really bass heavy or complex music, the resistance in both speakers could conceivably drop to 2 ohms at the same time, in which case the amp is seeing 1 ohm....not good unless the amp is rated for sustained 1 ohm loads.
I, personally, would not drive both pairs of speakers from the same amp. Besides probably not delivering the best sound, there's just too much at risk (amp, EMITs). You'd be better off adding y-cables to your pre-amp outputs and running a second set of IC's so you can use two separate amps to drive these fine speakers.
