2 Person mechanics ?
 

R1 on first, ground ball to infield. Initial play on the batter runner going to first base. R1 sucessfully goes to second base and takes a turn toward third, F3 throws back to second for a pick off attempt on R1. Whos got the call on the pick off attempt at second?

Base umpire has almost all calls on multiple plays in the infield. About the only one the plate ump takes is when a runner is on 2, the first throw goes to 1st and then the 2nd throw comes back to 3. On that play the base ump would be moving into the infield and 3rd would now be behind them.

BU has everything at 1B and 2B. PU would have leading runner at 3B in this scenario

Thanks for the replys.
Another question on pick off attempts. Whats the rational, other than a possible better angle with the PU taking the the pick off attempt on a batter runner rounding first.

BU has primary responsibility for play on BR returning to 1B. (In fast pitch, pick offs are attempts by the catcher to throw out a runner who has taken too big a lead on a pitch that is not hit.) Depending on the placement of runners ... and thus, the starting position of the BU ... the PU may indeed have a better angle for a play on the BR returning to 1B, but it is still the BU's responsibility.

Speaking ASA and NFHS:
There isn't any the BU has this play, PU will watch and offer any help possible if asked after the play, but the BU has the primary call on that play.

Glad you un-retired, Dave. :D:D