Whatever you do don't make the mistake in buying two different kites from different manufacturers. Even two different kites from the same manufacturer will be different. For example a 12m Xbow and 12m SB from Cabrinha or 13m PD3 and 12m Mr F from JN Kites or 13m Fuel and 13m Rev2 from Slingshot will have different wind ranges and handle differently.
Basically keep it simple and try to get the same kite in the same brand but different size. You'll have happy kiting that way.