Air, like any fluid, will always take the path of least resistance. If one branch tube is shorter than the other or one airstone placed shallower than the other or one airstone smaller or even newer than the other you might not get any air out of two airstones. This is especially true if the airpump is barely powerful enough to handle two stones. I usually don't use simple branch Ts any more. I only use airgang manifolds that have an inlet and valves for each outlet. These can usually be adjusted to power two stones.