O'K, for example you have two PIR, like this http://www.sparkfun.com/products/8630
. I'd attach one of them to the wall ~1m from the flour,
and second one right above the first. Than piece of cardboard or wood 30 x 30 cm should be inserted in-between to separate vision fields. Other option, use a desk surface, mounting one PIR on the desk and another below or under. The idea, that adult human will always trigger both sensors, though short "waiting" delay could be set in software, to leverage difference in time response. When 1 PIR trigger, software could wait 1-3 sec if second trigerred as well, before make a decision animal?human.