@xaral : I'm working on a rather similar solution (motion detection). I've tried both sensors: PIR and US. I've made different tests and finally dropped the US for this use case. It is not the right sensor for your job and let me explain at least one of the reason, which in my case was the main.
The US sensor and the PIR sensor have really distinct detection area. You will end up with having one sensor detecting a motion and the other not, and vice versa. In my particular case, I had to be in a really narrow area to be able to detect a motion with the US sensor whereas the PIR sensor was detecting the motion...
If you want an advice, forget your US sensor for any motion detection solution and focus only on PIR sensor(s). To avoid false positive, keep your two sensors, choose a position where their detection areas cross each other and only trigger an action if BOTH detects a motion.
Good luck.