One act, two fouls?
 

My BJ and I (LJ) both threw a flag on a downfield pass play. While conversing with R, we learn that I have DPI and the BJ has Def. holding on the same player (BJ saw a foul before the pass was thrown; the player continued to hold after the ball was in the air which is when my flag came out). My question is, do we signal two fouls to the pressbox and give A more options or, since it was one act, do we just signal DPI?

No harm if you did signal both fouls. Some sharpie could say how can the BJ have a DPI before a pass is thrown.

For an NFHS game, I wouldn't even bother to let Team-A know about the holding as the DPI is 15 plus the Auto-1st down.

If it were an NCAA game, since DPI is a spot foul, a holding foul could get them more yards. Both would be auto-1st. In that case, give team-A the choice after explaining which gets them more should they want that.

On this same note, let's say you have DPI and the way he interfered with the receiver was by grabbing his facemask. Which foul do you go with?

DPI because it's an automatic first down.

If he grabbed his facemask to the extent he committed another foul, you clearly have the personal foul version. The 5 yard version is only for incidental grasping, and if the contact was incidental, it isn't DPI.