I think it depends a lot on how exactly things happened. It depends on how much time passed from the stoppage of A, the speed, and many variables. It is difficult to answer just with such description.
In general, if both players are calling a foul it should be treated as an offsetting foul. However, if the players had no chance of catching the disc, the following rule would apply : “17.9.2.1. However if this occurs after the disc has been caught, or after the relevant player/s involved can no longer make a play on the disc, this must be treated as an Indirect Foul (excluding contact related to Section 17.1)”