I was wondering if two PIR motion detectors generally facing each other could set each other off? They are not directly in front of each other, but in the general detection view. One is professionally installed and the other would be a DMMS1.
Nope won't set each other off
Funny that you mentioned the DMMS1. I have one in view of a first/Kickstarter-era ST motion sensor, but in adjacent rooms. The DMMS1 flashes a (large) LED when motion is detected, and I think that flash (perhaps its reddish color) is enough to trigger the ST sensor if both rooms are dark (presumably becsuse then it's visible enough to trip it).
Or it could be the lights from my Christmas tree, triggered (if there wasn't alredy motion nearby) by the sensor in the room adjacent to/before that one. I'll find out if it keeps happening after I take it down. I don't think I moved the tree after I moved the DMMS1 to its current location, but we'll see. 
If it is the LED, it's probably able to be disabled by a Z-Wave parameter, but I like the LEDs now since I've found they're an easy way to tell if the batteries are dead or there's a problem with the device vs. something with my automation or (mostly when I was on another platform...) the hub. As Mike said, a mere IR sensor itself shouldn't be a problem.
My first reaction was "no way" but after a long think... I moved down to "I can think of a way" 
PIR is Passive Infra-Red and the sensor is actually two sensors on one slab of silicon. The real "trick" to them is the Fresnel lens that "shatters" the incoming light into multiple directions. The intent is to take tiny adjacent slivers of the scene and have them each hit a different side of the sensor. That way it detects one side getting (instantaneously) more IR than the other and it calls that motion. Static IR that hits both sensors are ignored. Slow moving (shadows) IR balances out and also gets ignored.
LEDs are very narrow frequency emitters. A Red LED is unlikely to produce any IR. It's flash should be ignored / invisible.
Now comes the devil's advocate side:
The LED is unlikely to emit IR but the IR sensor could (especially when turned to it's highest sensitivity) detect RED as "close enough" to IR. In other words, while RED is not IR, the particular PIR sensor used MAY be not so much an IR sensor as it is a red+IR sensor. The tiny piece of glass at the surface of the PIR sensor is supposed to filter out non-IR... but maybe someone decided that the right glass was a good place to cut costs. 
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If I do manage to set something off it will probably be at 3 am.
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