All interested:
I had some unrelated setbacks affecting my available time to experiment on my long term solution. Running the batteries in parallel was just my idea to learn if I could resolve the "sag" hypothesis with items on hand; it was not intended to be a final solution. If the sag (LV cut off) condition is eliminated then I can focus strictly on the power supply/battery. However, if the problem persists then it may still be a supply shortcoming of the LiFePO4 cells, or its then the shortcoming of the controller. Either way it costs me nothing to start with the parallel battery testing. I am eager to start building custom battery packs and while I've learned the LiFePO4s are not ideal for the ebike application, I do have (nearly new) three 48v 18ah LiFePO4s by which to (safely) experiment with...otherwise I'll be looking to sell them used on eBay. I'm not committed but my plan is to break them into two new custom packs: 52V 20Ah+ and only run one "new" battery on each of my two 1500W fat tire bikes...and pray that's enough to give me the performance I'm seeking.