Well yes with two of these on Mach 3 you can swap programmes be tween each machine.
If its not homing it will be a sticky limit switch probably on the z ...that's my guess anyway
There is only 350 or so parameters on the tnc 355 and half of them are not used..you can run a modern drive in speed mode so it uses the encoder as a tacho, then use the old encoders that go directly back to the Heidenhain for position reference and have the drives set for the analogue command +/- 0 -10v from the Heidenhain. If the encoders are coupled directly to the ball screws its much better for accuracy..the tnc 355 with modern drives will work better and be more solid than any Mach 3 set up but I'm an Heidenhain fan..each to their own
Look forward to the build hope it works well!
John.