Tying PLC 0VDC terminals together?

I'm working on an application that uses 2 Siemens PLCs, both powered by the same 24VDC power supply. This application also uses an input device (light curtain) that has both a 4-20mA analog output and an NPN transistor output, also powered by the same 24VDC power supply. The analog signal is connected to one PLC, and the NPN signal connects to the second PLC.

Would it be a good idea or bad idea to connect both PLC 0VDC terminals together, or totally unnecessary? I am having problems with the analog signal dropping to 4mA when the NPN signal turns off, and I feel like it's a grounding issue or some mis-match in the 0VDC plane.

(I'm not an electrician and I have near zero experience with PLCs.)
I'd really appreciate any explanation someone might have to offer.

http://accautomation.ca/heres-a-quick-way-to-wire-npn-and-pnp-devices/
The above link is a quick overview of wiring NPN sensors.
NPN sensors are wired with the positive on the PLC input common.
You should not have to join your PLC 0VDC terminals together. Your powering up both of these PLC's with the same power supply.

I would start by disconnecting the NPN signal and operate the light curtain. If the 4-20ma signal is still not working correctly, it must be the sensor itself.

Hope this helps,

@gclshortt: Even though everything is fed from a common power supply, I still insist on wiring all commons (or at least one common) to every PLC module, drive, etc. that has a common terminal.
Even on some drives, I'll wire several commons if the connection point is a removable terminal block.

That insures that there is always a 0V reference present, even if something else is unplugged, as is especially common (pun not intended) during startups or troubleshooting, where I begin with all pluggable things unplugged.