My bet is that, since soon as you plug P2 display in it stops the segments on the master display working.. have i got that right?? The fault will be the P2 display glass itsself. had the exact same problem with my Stellar Wars when i brought it home. went thru all the mucking around changing the 10k resistors only to find them burning again, then discovered pulling the connectors to the P1  and P2 displays all the segments came on, on the MDB. i had to replace TWO glasses, as the were both crook, burning different resistors and stopping different segments from coming on. Somehow the glasses were pulling the segment line down, burning up th resistors and causing segments to go out on the MDB. Replace the P2 display..

sounds like your lucky to only have to replace one display,, unlike me, was an expensive exercise!!