[Kevin Sevcik]

I'm far from an expert on these things, but isn't one of the legs referenced to ground or neutral or something? I didn't think most garden variety switchmode power supplies were isolated. So if they're not isolated, the common on supply A is going to be at a similar potential to common on B, with some sort of electrical path between them. So connecting + on A to - on B will be similar to a short. Or that's how it works in my head for non-isolated power supplies. I'll agree that you can treat an isolated power supply like a huge battery for voltage generating purposes.

I suppose you could check by checking voltage between - and ground and - and neutral on both supplies. If you get some sort of steady, not-floating value, then you'll want to be careful when you hook the two supplies together.