Golden Cheetah has the ability to track 2 PM at the same time (PM and Alt PM) but it is not able to write both PM Data into disk :-(
I'm also too cheap (currency exchange is 4x) to use DCRainmaker's Analyze tool.
I'm gonna try using an iphone + ipad to connect to the Zwatt & the Assioma using Wahoo's iphone app to track the data to compare.
If you can capture the data stream from both PMs using two different devices (your head unit, plus either directly into Golden Cheetah, or into the phone or Ipad) you can import both files and compare them in GC. Click the "Toggle Compare Pane" button on the right hand side of the GC header. Then drag both files into the compare pane at the bottom of the screen.
ETA: You'll also need to click the "Off" button on the right side of the compare pane to turn on the comparison feature.