Say you have two dead ball fouls, yes it will come back to the same spot, but my question is, if the first dead ball foul is on the def. does it give the A team a first down. then you mark off the 15 against A team, first and 10...because you haven't set the chains....
SIT: 3rd and 8 runner is tackled short of the first down. after the [play Def late hits, then A team late hits B, you know the Def was first. Do you mark off the 15 first giving team A, a first, then mark off 15 back. 1st and 10....

Yes it would be a 1-10. Read Case Book 10-2-4 Situation C page 78.

Had this last Thursday night, JV game. Runner OOB, short of the line to gain. USC on B, then a USC on A. Mark'em both, 1st & 10.

How and when do you give the signals?

I think that some will say signal both and then mark them off and others will say signal one and mark it off and then signal the next and mark it off. If you are working without a microphone then I would signal both fouls initially. Then make sure about acceptance by each team. Then signal the first foul and mark it off then signal the second foul and mark it off. If you have a microphone then you can signal the acceptance of both fouls and communicate both fouls while U marks off both fouls. Just make sure however you do it that it is clear to the fans and announcer what you are doing.

You should also keep someone or something at the first spot so that you get back to the same place if both fouls are the same distance.

I initially gave preliminary signals denoting the PFs against both team. After consulting with the captains, I gave the PF signal against the offense then the umpire walked off the penalty. I then gave the PF signal against the defense then the umpire walked off the penalty. I blew the ready for play and continued the game.