I believe you cannot use the same pin to trigger both sonars, as they will ping simultaneously
and the waves will interfere. You also cannot let them free-run, as they will again interfere.
And if you use separate trigger pins then you might as well use the pulsewidth mode, rather
than the analog mode, as things will go faster. Most of the time, you will get a pulse out in
maybe 10-20 msec. With analog mode, as I think you know, you have to wait 50-msec for the
analog signals to stabilize.