Thanks... I haven't thought about pinging the ultrasonic sensors alternatively. It might work. The thing is that I don't want to put them so apart, maybe at a slight angle from each other (of course facing other direction), but next to each other.
I need to count the people entering and going out a door... As a condition only one at a time in a single direction and with enough distance between to register as different. Probably the beam interruption is the best since it offers the best detection rate. The Genie kit seems pretty expensive and large (at least from pictures). Do you think it can be replicated with a photocell (infrared) and an emitter (I don't know if one exists for android)