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.
Thanks. Finally figured out how to get to the compare pane and actually seeing something. ("OFF" was the key thing to press)
I'll do more comparison when I get the chance and also compare it outdoors.
I actually did another test this morning but the data from the F3 (Assioma) was full of dropouts so I won't use that data. (so, both the Assioma and the Zwatt has Power Drop issues and placement is key. On the Zwatt, i usually place my F3 on the ground underneath the crank for close ANT+ signal. Yday (the below data) I had the F3 connected to the Assioma and place it on a stool Beside the pedals and all was good in terms of data dropouts, this morning I had the watch on top of the laptop in front of the handlebars and it gave a ton of dropouts.
Looking at the data, it's clear that GC just plots them one on top of each other w/o actually lining up the data properly. Well, it can't actually cos there's no 'same data'. I usually start the watch earlier than GC hence you can see the delta esp in those 30s intervals not lining up.. but the delta is there.
Pink is Assioma (connected to Fenix3)
Green is Zwatt (via GC on Laptop)
Critical Power Comparison. (don't mind the poor FTP performance)