no you dont need the same exact card. one could be from evga and the from bfg. memory does not need to be the same either. but keep in mind that the smaller amount will be used.
if one card has 1gb memory and the other 512mb... both will operate with 512mb. same thing goes for core and memory speed. everything thing will run at the slowest components speed.
The cards can be of different speeds and the card will operate at those speeds.
It's a simple fix by overclocking one to match the other, but they will operate at different core/shader/memory speeds with no problems.
I have a stock 280 and SC SLI'd. SLI FAQs