I guess it's hard for me to see the confusion. I just assumed since it's the same reservoir, the one, and lowest drain is the most effective, and any filter that touches that same fluid, should be changed at the same time. At least that's how I did mine. Since the HST fluid is the transmission fluid, the intervals and quantity in the manual would apply to the HST
It looks like one drain would allow you to keep some fluid, if only changing one of the filters, since the drains seem to be on separate levels, as are the filters.