Stick with your 770 for now (it's an awesome card after all) and then get another one if you need to. Best to stick to having one powerful card than two weaker ones.
Yup. Drivers tend to be more stable with one card, games sometimes run better (due to drivers), less power consumption, less heat, cheaper to buy usually, less space used up in your case. The benefits go on.In almost all cases one more powerful card will be better than two less powerful ones.
BF4 system requirements have yet to be released but, since it's running on the same game engine, expect similar requirements. If I were you, I would wait for a good sale, then get another 770.