I'm sure people here would be willing to be a medium to help you trade from one of your games to the other.

You have a Charmander on X, and you want it on Y. I come in "hey, I'll help you. Just give me your Charmander, and then get on Y and I'll trade it back."

Just make sure you ask for collateral. That means if you give them your Charmander to hold, make sure they give you something equally rare so they have a reason to give it back to you and not just keep it.

Alternatively, I'm sure there will be plenty of people around here willing to trade if that's all you're concerned about. The online trading has really simplified the process. When I was playing the DS versions it was insanely easy to find the Pokemon I wanted. A lot of people specifically breed pokemon and give them away freely. I managed to get all the starters from gen 1 without having to give away anything of value.