Any time you use more than one power supply they must be phased together. There is a Lionel Corporate video on YouTube "phasing two or more transformers to be used together and why" (or close to that title)
Mike isn't boring and this is a great "101 primer"/refresher for basic electrical theory imo.
Note if they work, they are most likely in phase because of how the new units are designed. Note a few of the modern ones got the wires reversed internally at the factory so checking phase isn't really redundant. (and knowing WHY is useful anyhow)
You can add another power supply via lock-on; but are you considering tjhat you need to set it up as a new block? (nonsense is courtesy of "spellwreck"; not human error... I fixed what it let me) . Stacking power supplies in parallel (increases amp feed) isn't really ideal. Blocks matched to it's own supply is better for most folks. (when you run 7-8 big motors on one lighted train(ABBBA) you might run parallel...just maybe)
You can use one common bus for all supplies. You must add the supplies max amps together and the fat bus ground must carry that total amp load back to say a ground bar, then smaller leads to each supply to the bar. I.e. the common bus should be as fat as if all the hots got tied together as one fat hot. (you're looking at a 10g or larger common and maybe 14 hot bus, 16g drops (or similar... check against AWG wire selection charts, all over the net 
) Remember oversized wires don't burn as easy; too small is bad 🤔
