My solution would be to modulate each transmitter with a different audio frequency, and then at the receiver employ two audio filters to separate them. This way, even though the signals may combine in the shared air medium, you still will be able to separate them with filters, much the way a graphic equalizer can separate music into frequency bands. I've done similar things with LEDs and lasers. The only difference here is that the ultrasonic transmitters are likely around 40 kHz, so you'd want to keep your modulation frequencies in the low-middle audio band (I'd recommend between 250 hz to 800 hz). On the transmit side, I'd recommend a strong simple modulation, such as totally cutting the output at the modulation frequency. On the receive side (and maybe on the transmit side too), you might be able to simplify the circuit design by employing some DTMF (dual tone multi frequency) encoder/decoder ICs, as they have been around since the advent of touch tone telephones, and you'll have a lot of parts to choose from.