Actually, there will be no way of doing it, as you need to have memory to flash GBA rom, and slot 1 to run NDS ROM, so that way they can connect and make a link...
The only way of accomplishing this, is to have a memory (slot 2) and ROM (retail game, or dump in slot 1)..
EDIT: It is possible if you have a original GBA game, but if you plan to use ROMs, than slot 2 is your only option..And why don't you buy EZ 3-in-1, it's very cheap..