Polo Rules

But you have to be very careful to not have two different decisions made, the two refs might see the same thing, and make different calls.

So really you have to clearly define who can call what.

+1. Having players argue with the ref is bad enough, but having refs argue would be worse. I am all in favour of line refs getting help from goal refs, and try to encourage it, but final decision has to rest with the line ref. On the other hand, a line ref that decided to ignore a goal ref who had a better view of a marginal call would have to be very, very sure that s/he was right.