Final Fantasy I & II (already often conveniently packaged together!)

Final Fantasy III & IV DS

Final Fantasy V & VI Advance

Final Fantasy VII, VIII & IX (PSX extreme!1!)

Final Fantasy X & Kingdom Hearts

Final Fantasy XII & Kingdom Hearts II

Crisis Core -FF VII- & Final Fantasy Tactics: War of the Lions (PSP...woot)

Chrono Trigger DS & The World Ends With You

Final Fantasy Tactics Advance & Kingdom Hearts Chain of Memories (GBA fun?)

Revenant Wings -FF XII- & Final Fantasy Tactics A2 (more strategery on teh DS)