It depends on what you mean by "communicate."
If you trigger them both at the same time, then when unit A receives the signal from unit B, it will tell you the half distance to unit B (and vice versa).
Unfortunately, if unit A happens to receive a reflection from its own signal before the signal from unit B arrives, it will tell you the distance to the reflecting object, not unit B.
If you only need "one way" communication and don't need the sensors to be waterproof, then you can fix the reflection problem by using HC-SR04 sensors and removing the transmitter transducer from the "receiving" unit.