Well, the TPC 400 handled the parallel connection and output the 20amps. The Legacy Powermaster 360 of today is its replacement.
I am surprised that more forum members haven't chimed in - there are a few more forum members that operated dual motor Pul-Mor ABBA power sets with incandescent lighted cars using two PH-180's.
Another variation that could creep into the equation is that the "new " production run from perhaps 2015 forward is different internally, at least with respect to the breaker, maybe more. A mystery story as to why it happened, but it did. I believe that they can be identified externally by part number markings. (not positive on this)
Are you sure you want to do this? This is entering welding territory when a short occurs - no more sparks