Can multiple 2-way occupancy motion sensor switches be used together?

Yes, you can, under code, but you will also need to have 'real' switches also installed. As far as the second part write on a bit of paper, in marker, and stuff it in the the motion sensor junction box.

Will both of the sensors cover the entire stairway? This is the game changer.

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.

Lutron Maestro line of switches can communicate wirelessly to multiple ceiling mounted occupy sensor that use a battery.

The switches work the lights as normal if the sensors battery dies.

I've installed them and give it a thumbs up.

Battery Version

This is a perfect situation for smart switches as mentioned in another answer.

Small suggestion regarding labeling - I wouldn't put a label on combustible paper inside an electrical box. Use something made to be in there. A little electrical tape flag with PARALLEL written on it, on the wire nut, might be a good way to go.

Have successfully wired 3 Lutron motion sensors to control a common light fixture so that light would be activated regardless of point of entry into room, hall, etc.

There no code (NEC) preventing the output from motion detectors on the same circuit from being combined. I did this at my last home for a very long circular drive, 1 detector at each end and 1 in the middle it is still working today almost 20 years later but 1 of the sensors did require replacement after a tangle with a weed eater. To wire this up I used 12/3 with ground the motion detectors get the hot(black) and neutral (white) at each location then the lights are tied to the red (switched output from each detector) and the neutral (white) to each lamp , grounds to the fixtures and junction boxes. Works great, for a stairwell you may want a motion switch with override, I think this may be one of the areas some one may have a problem with automatic only. My plant had areas of automatic only lighting that had to be updated with Manuel overrides.

As far as changing bulbs, LED moots that issue. You can also use LED fixtures which are bulbless - the LED proper will outlive all of us, however quality matters in the electronic driver.

For that matter, one option these days is use a 4-5 watt LED and simply burn it 24x7. This sets you back about a dollar a watt a year, so it's not a crippling cost. At that point it does not require a light switch at all, but it must still be designed so someone does not turn it off thoughtlessly or carelessly.

Marking wires for "the next guy" is a good idea. However you can imagine what will happen: a paper note will be considered trash, and instantly get lost.

Your best bet is to mark the wires directly. You can do this with colored electrical tape. There is a risk of (especially cheap non-3M tape) getting warm and unwinding off the wires, leaving a sticky mess and no useful markings.

To avoid that, use shrink tubing (the hair dryer will need power, don't use a match, it doesn't shrink evenly, and setting your house on fire defeats the purpose of careful electrical work!)