Unless you are not swapping, and have setup ISBoxer in a very specific way so the games on one monitor will never go to the other monitor, then they are probably rendering on one card.
On the other hand, if you have force set it in WoW as to which card they are using, then when they do swap, they still render on that card, and you then have two games rendering on a card, but displaying on a monitor not powered by that card.

In both situations, there is a performance hit, but, whatever is running your active region, is handling it already (which, from your description, is probably the 1060).

Two monitors and two cards is probably the worst config if you are going to have games swapping from one monitor to the other (because one is running the "main" region). That said, a single 2070 would remove any performance hit, and probably provide oodles of vram to spare. Under DX11, you need approx 500MB VRAM per game instance.