OK, I'll throw this out....
As you are north of the border, one of the Canadian members may input if this is allowed in Canada.
Tou can purchase an electronic KWHr meter from a company like E-Mon D-Mon, and install that to record anything. It uses split core CTs so there are no lines to tap, cut, etc. One meter can record multiple loads (sub-panels) via additional sets of CTs. Square D and others also make similar units.
You could do what I think you want ('summation') with one meter and two sets of CTs.
Or, I may not understand your intent fully.
Google 'E-Mon' & check out the info/details.
