We haven’t used MaxBotix on a GrovePi and I suspect the firmware would require modifications in order to support it. Even if we would support it (by having it implemented and tested), you’d still only be able to connect one sensor of this kind to the GrovePi.
Bottom line is that either way, you’d be capable of connecting just one ultrasonic sensor at a time on the GrovePi.
Did you know you can stack 2 GrovePis one on top of the another? Technically, this could allow you to run 2x Ultrasonic Sensors + 1x DI Distance Sensor, so this would make for 3 distance-measuring sensors. Would this help you?