Not at present, with non-matched cards. If you had two identical cards (and a suitable motherboard), you could use Crossfire (for AMD) or SLI (for NVIDIA) to combine them, as long as the cards themselves support multi-GPU technology. (I'm not sure whether NVIDIA's low-end ones do - I've got a couple of G210s from different OEMs in my hoard of spares and neither has an SLI connector.)
That said, there are experimental technologies that have been demonstrated in DirectX 12 which allow heterogeneous multi-GPU rendering. They probably won't show up in games for a while, though.
I'd use the GeForce 210 - I think the raw performance isn't much different but the 1GB VRAM will be much more useful than the 256MB on the Radeon.