Can small supply pipes be combined into a larger supply?
I'm not a plumbing code guru, but I see no problem with what you have planned. The only side effects I can think of are a slight increase in noise (due to the added turbulence) and the possibility of stagnant water in one side, should there be an odd flow imbalance. 

Be aware that the two 1/2" lines may not be supplied by a 3/4" line, though. They may just split from a single 1/2" line, whereby your plan would be pointless. A split 1/2" line is probably more common for bathroom sinks, which don't require a lot of flow.