I'm new to AGOT and our gaming group wants to have even amounts of cards. Is there any way to build up a deck for house martell and house greyjoy that are equivalent to the core sets?

Darksbane has a topic over on CardGameDB on some decks to build with just two core sets, and later in the thread there are some similarly built Greyjoy and Martell decks:

http://www.cardgamedb.com/forums/index.php?/topic/207-4-decks-built-from-2x-core-set/

They're not necessarily the absolute best decks you could build with that card pool, but they'll hopefully give you some ideas. First and foremost, if you've got the GJ and Martell boxes, obviously don't stick to core-only decks for the remaining houses, as those deluxe expansions contain a good number of plots and neutral cards you'll want to mix in, plus the Kings and Queens from KotS and Multi-affiliation characters from PotS. And vice versa, mix in core stuff with the GJ and Martell decks.