As long as the Motorola radios are in the same series, they are usually compatible. For instance: a XTS5000 can talk to a XTL1500 in digital and analog modes. This of course assumes programming and any encryption is compatible.
Now a XTS5000 is not able to talk to a MotoTrbo XPR4550 in digital mode. They will work in analog, but Motorola digital is not the same across the board.
This is due to XTS radios being FDMA, XPR radios being TDMA, and then there are spread spectrum frequency hopping radios like the DTR series.
There are a lot of details that can cause a radio to not be able to talk to each other in digital mode. Sorry to open the can of worms.