Hello Dave,
Let me start with my first recommendation: Have you considered networking the PLCs and messaging the necessary information between processors instead of cross connecting them with IO? That is the way I would go first, if you can.
I also recommend you get a copy of NFPA 79 Electrical Standard for Industrial Machinery. James mentioned some of the relevant issues - there are several. NFPA79 spells out that Orange (Not Yellow, although lots of people think that is the right color) is to be used for cross system wiring that remains energized, but it makes some exceptions for cables provided you adhere to some specific requirements spelled out in the code. The standard isn't meant to hog tie you. Look at as a guide to help you build quality systems. The handbook is about 40 bucks online and well worth it.
22AWG wire is smaller than what I prefer to use, but it is allowed. The circuit providing power cannot be fused at more than 3 amps. Pay heed to the 3 amp limit - that does not mean you can fuse an IO module at 8 amps just because its rated for 8 and use 22AWG wire just because the PLC module on the other end only draws 150mA on its inputs. You are limited to 3 amps because that is what the wire is rated to carry. All too often I've seen IO modules fused based on what the card is rated at, not what the smaller capacity wire can carry.
My personal preference is to not use anything smaller than #18 unless its OEM wiring already permanently attached to a device by the manufacturer, even for DC discrete and analog signals. (#18 is 7 amps BTW, so the #18 circuit cannot be fed by any breaker/fuse larger than that, no matter what kind of devices may be in the circuit)
The cable must also have insulation rated high enough for the highest voltage it will share a conduit, raceway, or enclosure with. So for example if it shares a wire duct with 480V conductors, even if it just crosses thru the duct, you have to have at least 600V rated insulation. Some of the instrumentation cables have insulation rated at 300V and some is even lower so check before ordering. Also, the insulation rating is the peak to peak voltage so if the cable shares an enclosure with 120VAC then it has to have a 300V rating, 150 isn't going to cut it.
Since you are running a conduit, without knowing more, I would recommend that you pull #18 orange wires - if that works for you.