It can be done an the Power Houses do function in parallel, but you do lose some level of protection and you do have significant amperage available and between blocks.
I watched a layout run that had several block along a 1000 square ft plus layout in a house attic. Up to 5 or 6 PH where in parallel in each block, because consist ran with up to 4 dual Pulmore motor engines in MU on each block. At time you could have 2 Consist in the same block.
If you had a derailment you had to act fast and yes there were a lot of sparks. Otherwise it ran fine, but this was not a layout you could walk away from, and it certainly had potential for disaster. I don't think 2 PH in parallel on the same block is much of an issue. No different then 1 on 2 blocks when a train is crossing the blocks or they are thrown in parallel via switch when moving between block.
I do think the TPC is a method to improve the safety though. G
