When you SLI two different cards, the weaker one will set the pace. For instance, the EVGA has a core clock of 1111mhz. The MSI has a core clock of 1137. Therefore, when you SLI them, your current MSI card will underclock itself to match the slower EVGA 1111mhz. A way around this may be to overclock the EVGA card, but if it cant maintain a stable core at that speed, you will end up crashing. However, having two 770's in SLI would surpass the performance loss from the underclock.