Phasing a Z-4000 and Z-1000s

The Z-1000 set to 10-12V is a chopped sine-wave.  The Z-4000 output is a relatively pure sine wave.  If I understand correctly what you did, you're sort-of comparing apples and oranges.  The meter shows 0 between the Z-1000s because it is the same type of AC signal.   Because the waveforms are materially different, phased chopped-sine and pure-sine supplies at the same voltage will show a non-zero difference between the hots.

Turn both throttles all the way up and try it. Pick the polarity that gives the lowest voltage and go for it. As Stan2004 says, you will never get a complete zero between those two transformers, but it will be easiest to see at full output.

As long as the Z-4000 is for accessories only, you will be just fine!

Ok did not realize that the transformer output waves were different. Since all share a common return, I was trying to have this nice synchronous system. I will give it a try tomorrow with the throttles full tilt and do some measurements, it will be interesting to see how it comes out. I may even compare digital versus analog meters.