I had that same concern, as I have mine very similar. Batt1+ via ACR to Batt2+, and Batt2- to engine block. Since the current sense is only on Batt1-, then when Batt1 needs charging it could potentially overcharge Batt2. But when Batt1 *is* charged/low loads, then Batt2 could be *undercharged* depending on its state of charge. So not ideal, but appears OK. Charge voltage at Batt2 starts off around 14.8-15.0, then will drop to ~14.0 after a short while. High voltage concerned me since I went for an AGM/deep cycle battery which dont like being over 14.6-14.8 for any length of time. Fingers x'd it'll be fine - plus I have a 3 year warranty :-)
