[@The One] Hmmm while I don't mind twins sharing a summon so much the fact of the matter is that each summon has one element. I designed them that way so that each feels powerful and powers generally match element. To mix too would require me to completely balance the system around people getting two elements. 

The way I currently have it written, a dark and light type elemenal would be unable to be destroyed have illusion based magic, enchanted combat weapon, and a weapon for the summoner. Which is a lot of powers to get just off your elemental types alone.