so far there is no NLE that boasts any advantage from SLI or dual graphics cards.
BUT..... Avid Liquid 7 is optimized for GPU, and so is the new Premiere Pro 2. This means these apps willt ap intot the power of your graphics cards. DO they or can they tap inot 2 cards???? Don't know for sure, but I have a hunch you would see some things accelerated as a result.