Two man mechanics ?

[hit4power]

Two man crew in the following situation: R1 on 2B, R2 on 1B, 1 out. BU is over behind SS. Fly ball hit to deep centerfield, F8 makes the catch and R1 tags to advance to 3B. Throw comes directly to 3B and R1 slides in...

Question: who has the call at 3B and where should that umpire be positioned?

What actually happened: BU remained in his original position behind SS. PU came out from behind the plate when the ball was hit but did not move more than a few steps down the line to 3B. The throw was a little to the plate side of 3B and the runner slid in as F5 applied the tag. From behind SS, the BU clearly looked at the PU to see either: if PU was going to make the call or perhaps indicate what he thought. After a perceptible moment of staring at PU, the BU signaled safe. DC was livid not only because of the safe call, but because BU was clearly looking at PU for help. It did not look like good mechanics on the part of either ump - what should have happened?