Yamaha keyboards have this ability, too. Some models call this the "Dual" voice option and refer to the two layers as the "Main Voice" and the "Dual Voice." But other models-- including the two you mentioned-- refer to the two layers as the "R1" or "Right 1" voice and the "R2" or "Right 2" voice.
By the way, if you're thinking about buying a PSR-S650 or PSR-S750 then you might want to wait a little bit and get a PSR-S670 or PSR-S770 instead (or a PSR-S970 if you can afford it), because those are the latest models and they've just been released in several countries. They don't seem to available yet in the USA (if you're in the USA), but they should be available very soon now. On the other hand, if you feel like you don't need the "latest/greatest" model, you could probably pick up a new or used S650 or S750 at a reduced price, since stores will be trying to get rid of their existing stock to make room for the S670 and S770, and people will be trying to sell their older models so they can move up to the S670, S770, or S970.