I imagine the problem wouldn't occur with both legs paralleled as they would tend to even each other out.
I never did get good stable voltages from this ST. I've gotten them usable by connecting them to a 3KVA transformer which balances them, but at a cost.
When a heavy load on one side would normally throw the voltages way off, the transformer will balance the voltages, but get very warm (wasted power).
I never was able to figure out what was wrong, but it kind of works. I still plan on running the 4 60v legs through a bridge rectifier/filter to feed my charge controller/inverter, but other projects have taken priority (such as rebuilding the engine....)