While not specifically supported I don’t see why you couldn’t have multiple battery systems within one property, as long as there was a way to manage the priority of where excess electricity goes. Possibly on different circuits or consumer boards with separate CT clamps.
For instance it is possible to configure the use of Zappi or Eddi devices to only use excess solar once it is over a certain amount. It would also be possible to have an automation wait for a certain level of export for a certain period of time before triggering an action like changing.
Presumably, should it ever come to market, the Givenergy power diverter could also charge another battery, as is says it can divert power to other devices.
https://www.givenergy.co.uk/pdfs/products/PRODUCT%20BROCHURE_FINAL%20(ONLINE).pdf
Getting it all working or an installer prepared to do the work might be a another issue…