[cowbyfan1]

I had the same situation earlier on a thread. Can only treat it like being the BU with runners in 1st and 2nd and a fly ball to right or center. Get the lead runner then look back ad R1 and if he is close then he tagged properly.
Key to this, for me, is giving R3 a look to make sure he was on the bag to begin with. Get the catch then relook at him and if he is close he is good. If he is 3 or 4 steps off then he split too soon.

Another thing that has been suggested to me in this situation is BU takes tag of R3 and even R2 while PU gets catch/no catch fair/foul going up the line some but to do that you need a good pregame. Kind of like R1 and a roller up the 1b line or fly up the rf line. PU should holler "I got line" letting his partner know he needs to take runner to 3rd. I realize this is an unusual suggestion but maybe a mechanic worth looking into.