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So long as they are the same base chip (ie 970) with the same amount of memory, there is no issue with that.
Clock speeds do not have to match, SLI will match the cards up to the slower of the two.
So everything is going to be okay if I use an Evga Gtx 760 4gb ftw and an Asus Gtx 760 2gb it will combine the memory and ill get 6gb total or 2 gb cuz of the one is low memory?