on 12/01/2019, John Rumm supposed :
Not sure you can really call it a limitation of the design - at least they
provide the facility in the first place, unlike many.

..All it would need to enable a split temperature, is 2x call inputs for
heat instead of the single one it has at the moment. Plus the two
temperature settings in the software, to replace the single
temperature. A fairly simple fix at the design stage.

Needless to say that precludes the system running Y plan with the valve
in mid position, and so favours modern fast recovery cylinders.
(if you program CH and DHW to be active at the same time, it squeezes
reheats of the DHW in between periods where the CH is satisfied
(perhaps with priority to the DHW)

That would not usually be an issue here. The DHW is usually brought up
to temperature in less than 20 minutes. 20 minutes with no CH input
would not be noticed.