Electronic – Trying to understand directivity of an ultrasonic sensor

You cannot change the frequency because it has been tuned for specific frequency, not to mention the legalč stuff even if you could. For given frequency (or better said wave length) there is a relation which determine how far the so called 'near field' extends. And only within this near field passive RFID system could possibly work, because it works similar to the transformer /the energy 'taken' from the primary side indicates the presence of the tag in the field/. The antenna near field extends to 'wavelength / (2 * Pi)'

So if you made the frequency higher, you would actually shortened this reactive detection range. Long range systems (10 m or so) works based on the back scattering principle, much like the radar.

The URM37 appears to be a typical "ping" module and does not have two sensors, rather it has a transmitter and a receiver.

You could build a system with one transmitter and two receivers if you got into the details of amplifying and detecting the signal yourself, rather than relying on the module to do it.

You might be able to slave additional modules to the primary one by removing their transmitting transducers and firing them all at once, though the circuit might misbehave without some loading.

The easiest solution would be to use multiple modules and take turns firing them so that they don't confuse each other. Just take the normal measurement interval and double it so that you are taking a measurement at that rate, but each sensor is firing half as frequently.