Displaying the same album across multiple box sets

[simbun]

I have both the Beatles Mono and Stereo box sets and getting them to display how I'd like is giving me a headache.
The album 'A Hard Day's Night' exists in both, same disc and track numbers.
My current tagging has both Albums as 'A Hard Day's Night', with a custom tag BOXTITLE of 'The Beatles in Mono' and 'The Beatles'.

I'd like in the control point to have them appear under 'Beatles, The' as a single container of 'A Hard Day's Night', and then to somehow seperate them at the next level, the problem is the discnumber is the same so I can't set discsubtitle or group.
Given I believe MinimServer is using album and discnumber as the top level grouping element (and that I wanted to maintain album) I was hoping to change discnumber (in MinimServer) to something that would differentiate the two, like appending Boxtitle to it, but it doesn't seem to work. I've put Discnumber.merge={Boxtitle} under TagValue but I still get the same errors.

Error: group contains tracks that aren't consecutive: Stereo Remaster in album A Hard Day's Night
Error: group contains tracks that aren't consecutive: Mono Remaster in album A Hard Day's Night

I know it's processing the tagValue as it complains if I change it to Discnumber.merge={BoxtitleXYZ} but maybe at a point later in the processing.

Any ideas on how this can be achieved, or how you would tag it differently?

Thanks