At times in the past on a large layout I paralleled a pair of fixed voltage 135 watt Powerhouses linked to a Track Power Controller[TPC 3000] for 270 watts to enable operating two long,lighted passenger trains in the same power district. Note the advice from Nicole regarding backfeed and, only parallel identical fixed voltage units such as the Powerhouses or "bricks" as they are often called. You are apt to have circulating currents as also noted above when using variable voltage units that are difficult to equalize [such as the pw ZW].
A better long term solution when running multiple long passenger consists is to dispense with paralleling and equip the passenger cars with LEDs in place of incandescent bulbs.
