Really hard to answer that as I know nothing of your system, but to answer that to majority of people no.
#1, best to stay with 1 more powerful GPU
#2, SLI is only really needed if you wanted more graphics power but limited budget, so your better off going for the same card if the only card your limited to upgrading is of similar or close performance.
Overall, No. SLI is has problems with comparability and micro-stutter and overall isn't worth it. It is much better to go with one better single GPU which has far less hassles and issues, as if the game doesn't support SLI, your running off one card that may not be very good.