They may do it but it's against all good practice for long term battery health. VW don't do it and neither does any UPS manufacturer (nearer the day job).
If they have fitted a changeover switch or similar so one discharges before the other I've not seen such a system before and I'd question it's effectiveness trying to squeeze an extra 20ah capacity (about half a day run time) Vs the dead simple pair of 75ah approach.
It's very common not to charge the starter on EHU as generally if you are putting no load on them from the habitation side they don't need it. Folks that need it are in either off grid a lot or store the van for long periods and make alternative arrangements.
If they have fitted a changeover switch or similar so one discharges before the other I've not seen such a system before and I'd question it's effectiveness trying to squeeze an extra 20ah capacity (about half a day run time) Vs the dead simple pair of 75ah approach.
It's very common not to charge the starter on EHU as generally if you are putting no load on them from the habitation side they don't need it. Folks that need it are in either off grid a lot or store the van for long periods and make alternative arrangements.