Two different ultrasonic range finders to minimize interference
The usual approach is to trigger only one sensor at a time. In other words, time-division multiplexing, rather than frequency-division multiplexing.

Otherwise, even if the sensors aren't pointing in the same general direction, coupling through the structure of the robot itself can create unnecessary interference.



Just to put some numbers with that: Ultrasound requires about 6 ms per meter of sensed distance. If you're interested in distances up to 2 meters from your robot, then each sensor needs 12 ms. If you have 8 sensors, that's a total of about 100 ms, which means that each one can take 10 measurements a second. This should be plenty for a robot that can move at up to several meters/second, depending on how quickly it can stop, of course.