We have two 3DS' & two copies of Zelda. I was wondering if there where any free games that we could play together. Or if we're just boned unless we buy a new game? & what, if, any, can you Cart Share? (or whatever it's called when you play multiplayer w/ 1 game)

Nope.

No free games, unless you wanted to play that AR game one at a time. or you could street pass each other until you have all the pokemon in that pokedex game. there used to be a free multiplayer zelda game on the dsi store, but i think thats over.

Star Fox 64 3D does that. You only need one cart between the two of you.

Pretty sure you'll be able to play multiplayer Mario Kart 7 off the one cart, when that comes out.

Can I have your other 3DS?

You can download Four Swords for free from the eShop. It's available through February something.

If you want there are a bunch of regular DS games that do this.