Are ultrasonic Tx/Rx pairs really just the same sensor?
Sometimes they are, sometimes they aren't. If transducers are sold as TX/RX pairs they are probably optimized differently and aren't identical.

Transducers do exist that can serve both roles so you can use two side-by-side to eliminate a deadzone, or multiplex just one for both transmission and reception. Or in your case, multiplex swap the role of transmission and reception between the two.