You cant, you can only SLI two cards of the same GPU. You can pair two 550Ti's, or two 560Ti's. Cant mix the models.
This should explain a bit.http://www.youtube.com/watch?v=XS5rBS8n-LM
EDIT: Dont be concerned about your PSU after watching the video, its a bit old and back then components did draw a lot more power than modern components. Your 750W will be plenty for a dual card rig.
Another thing he doesnt mention is the amount of VRAM you have in an SLI/Crossfire array. If you have two 1GB cards in SLI/Crossfire, you only still have an effective 1GB of VRAM. Both cards have to store the textures, the VRAM isnt shared among them. And if you have a 2GB and a 1GB card, it will default to the lowest amount, so you will only have an effective 1GB still.