Yes, you can wire multiple powerhouse supplies in parallel (and in phase - note some early powerhouses are not properly in phase with the polarized wall plug and require the Lionel 6SP-2983-010 that Lionel released to correct this). Some of the larger wattage rated TPCs, PowerMasters, and Lionel "transformers" do this internally from the multiple input terminals. However, it is much more advantageous, in my well experienced opinion, to have multiple power districts with common ground to spread the load. Also, be sure not to overlook proper over current and over voltage protections regardless of your power setup.
Furthermore, if you have, with consideration, split your track plan into blocks and power districs, you can then assign a PowerMasterr to a series of blocks for conventional operation while leaving other blocks available to be assigned to another PowerMaster for another conventional train or to be assigned full voltage for command operations. Add in rotary block switches and whatnot and you can get extremely dynamic with power distribution and block assignments. Or you can keep it simple and track/loop 1 is one power district with one PowerHouse and one PowerMaster and track 2/loop 2 is another power district with one PowerHouse and one PowerMaster.
