My two-handle bathroom sink faucet suddenly went weak on both cold and hot sides. I can get cold water or hot water (using H or C handle separately) but no warm water. Turning on Cold and then adding the Hot side (and vice versa) does not increase the flow from the faucet spout.
You may have a chunk of debris in your aerator, reducing total flow. If flow is restricted, you won't see any difference in flow between one faucet and two faucets turned full-on. So try cleaning your aerator.
If you don't know how to clean your aerator, see this link.
Even before you clean the aerator, you can probably get hot + cold = warm water by turning on one faucet just a trickle, then turning on the other till you get more of a trickle.