It's not all that uncommon to have female that go in heat around the same time...they tend to trigger it in each other...like humans, or so I've been told.
I personally would not breed two girls at once...ESPECIALLY when you are just starting out. One litter is a handful in itself...I can only imagine two!
And I'll be perfectly honest, I wouldn't be willing OR able to take that chance of unforseen emergency expenses times two!