It depends on your motherboard. If it has SLI support you can add another 760. If not you have two options:
1. Add a GTX 960 and use the 760 as a dedicated PhysX card.
2. Get a R9 380.
3. Sell your GTX 760 and buy a R9 390.
Option 2 will work better unless your games utilize PhysX heavily. Option 3 is if you are willing to sell your old card. SLI might outperform either option but considering the inherrant inefficiency I wouldn't be certain.