the battery to controller connection is part of the battery mount on the Semi-integrated battery bikes. try this guide to find it. https://radpowerbikes.zendesk.com/hc/en-us/articles/5723831771163-Semi-Integrated-Battery-Mount-Replacement?source=search
if you have an HS version of the bike you can cut the battery mount cable in half and splice in XT60 connectors to both halves. (The ST version the cable is too short to do this nicely, but you can order a new HS battery mount to use just for this mod.) Then you can run any battery with this bike as long as you have XT60 connectors on it.
I have 2 Rad external batteries in Parallel with the stock SI-Battery on my rover 6+, 3 batteries in parallel total, using the Datex3 connector from BGB.
THIS is where I have the question - is it simply as matter of running a cable (with adapter if necessary) from say my rack to the female part of this cable (see photo), or is there more to it? (again, to clarify, I'm just looking to swap as I ride, not run in tandem, although the tandem thing seems to be ultimately be the easier thing)
Assuming you want to run both at the same time, the best option in my opinion, from the controller connector run a wire to the parallel adapters controller input. then run wires from the batteries to the battery inputs of the parallel adapter. that is all that is needed.
I would highly recommend a parallel connector if you connect two or more batteries. if you do not use one then there is a risk of accidentally connecting the batteries at the same time and that can be ver bad, fire kind of bad, depending on the difference in charge state.