Is this your motherboard?http://www.newegg.com/Product/Product.aspx?Item=N82E16813130596
If so, your problem is that one of the PCI-E is X4, which means that in SLI, your cards are bottlenecked by this, as in SLI both cards will run at X4. There really isn't a good way to get good performance from any SLI setup on that board.
As I see it, you have 3 options.
1) Get a new motherboard that supports 8X/8X or 16X/16X for dual video cards
2) Sell both you 550TI's and get a 560TI or better.
3) Sell one 550TI and game with the other.