Don't. Sometimes it is possible to get two similar but slightly different cards to work with each other, such as a 670 and a 680, but the slower card will drag down the faster card and even if the frame-rate increases, it will feel like it's stuttering because every other frame will be rendered faster or slower than the other so the timing creates what is knows as micro-stuttering.
Do yourself a favor and go out and buy a new card and use that as a single card, while trying to sell your old card second hand. You will thank me later. Going the SLI route is just begging for problems.