Air in the plumbing problem
Air probably collects in the lines when faucets are in use at differing flow rates and then one is turned on suddenly. For example consider the upper faucet operating at a small trickle and the lower faucet is off. Then the upper faucet is suddenly turned on full blast. The straight through flow to the lower faucet from the forked fitting could actually act like a suction on the upper pipe for a moment and probably take in a small gulp of air from the upper faucet. 

If your fork in the piping looks like this:



You may want to try a fork that looks more like this:



. . .with the water flow from the tank entering as shown by the arrow. In this scheme there will be less possibility of the straight through flow suddenly starting creating a suction from above.