If your playing pokemon emerald on the DS, can you trade with a friend if their also playing emerald on DS?

Question

I might get a DS and sell my GBA

Answer 1

No. You can't trade between the two, you can only migrate. You'll need to put your emerald into the GBA slot and then diamond/pearl into the DS slot and on the menu after you get pal park, go to Migrate Pokemon and then select six Pokemon from any of your boxes and transfer them to your game (you Cannot trade them back to Emerald).
Once you do that, participate in the catching show and you'll be able to capture all six and have them placed into your box.

Answer 2

No, you'll have to do all the trading on the GBA as the DS doesn't have ports to connect to a GBA.