That's why the 16x ASDO project was first proposed several years ago, to allow for planned longer 16x formations in GWR's central region. But the pandemic overtook events, so there's less requirement for that currently.
16x ASDO is now being rolled out initially in the LTV/East region because although SDO isn't really required for the formations in this region, the system also includes OTCM (on-train camera monitoring, i.e., monitors in the driving cab displaying images from external cameras). All the platform-mounted DOO self-dispatch equipment (mirrors/monitors) in the east is life-expired, so having the monitors in the 16x cab means that there will no need to replace the platform-mounted equipment, thus saving £several million.
Having monitors in the cab also helps manage the risk of a PTI incident as they will remain active while the train starts away allowing the driver to make a final check (which you can't do with platform-mounted mirrors/monitors). Also, the ASDO part of the system includes correct-side door enable (CSDE) capability as protection against wrong-side door releases. So there are significant safety benefits even if the SDO part of the system isn't necessarily required.
Yes, same software, same database, and mostly the same hardware. But getting hold of a 769 to do some testing is much, much easier than getting hold of a 16x, so using the 769 as a test-bed helps the implementation on 16x to progress.