Can multiple 2-way occupancy motion sensor switches be used together?
For American readers: I cannot find any language in the NEC expressly prohibiting the use of switches (motion-sensing or manual) in parallel for ORed lighting configurations such as the OPs.  However, there's another problem, and that's user expectancy; even with manual switches installed, most people will see this setup and think "three-way switch", not realizing it's really been wired differently.  I, personally, would insist on having labels on the faceplates of both motion sensors mentioning that this is not a three way setup, and pointing up or downstairs at the other switch, as appropriate -- there's an off-chance it could get a clumsy bulb-changer bit because they only overrode one of the sensors, not both.

Considering that you have stairway coverage accounted for, and a way to label this, I'd go ahead with it.