in the HD-P2 settings, you have three choices "clock source":
Internal, Word Clock, or S/PDIF
In this case, because you want to sync the HD-P2 to a UA-5 > MT II combo, and the UA-5 doesn't have word clock inputs or outputs, what you will want to do is use the UA-5 S/PDIF signal as the clock source (but not the audio source).
So the overal gear chain will be:
mics/sbd/whatever > UA-5 > MT II
and
mics/sbd/whatever > HD-P2
you have the S/PDIF going from the UA-5 into the S/PDIF input on the HD-P2, and choose S/PDIF as the clock source. then choose the analog inputs as the audio source, so that the deck will only use the S/PDIF as the clock, not for the audio.
in regards to starting the two decks at the same time, it doesn't really matter. Because the two sources will have the same clock, there will be no drift in the two recordings over time. You'll have to sync up the two recordings in post, but you'll only have to do it once at the start of each set, as there will be no drift between the two recordings.