Well technically... You can, but that's only a possibility on motherboards, that have the Lucid Hydra chip on them, which allow you to sli/crossfire 2 completely different gpus(e.g. HD7870 and GTX660ti on the same mobo). That technology was first released in like 2010 and tbh it didn't make that big of a bang, mostly because it was all a bit too wobbly. Meaning, that sometimes you could get a performance boost in some games and sometimes it just crippled the performance and 2 gpu setup was slower than 1 gpu. It was(probably still is) a hit and miss, so you are better off just getting a single 760 and just selling the 550ti.