Assuming that you mean a second card that will be a Titan AMP edition, I think it should work, but the faster card will throttle back to the slower cards performance to syncronise the rendering. Same goes for memory. If one card has less memory the SLI config will use only the less RAM configuration.
So your SLI will be slightly slower than as if you had 2 AMP Titans but it should work as long as both cards are TITANS.