Polo Rules

a) maybe just point in the direction of play (like in football), as they will need to whistle at the same time.
b) and then maybe the football advantage call as well (just in case it's not clear who the advantage is for)
c) just flag/arm up I'd say
d) yeah, arms crossed, or waving horizontally.

In fact 2nd ref will not have a whistle, the aim is to find a way for him to tell the Main ref who will get the ball turnover in case if the main ref don't see it. (you see what i want to say?)