I think the best way to ask an umpire to request help of their partner or partners is to call time, ask the plate umpire for permission to speak with field ump and then approach them and quietly ask them to request help of the PU or another field ump only if they had a clearly better angle than the umpire who made the call. Showing up an official with barking and whining will typically lead the crew to dig their heels in and not be amenable to changing the call. Be quiet, be polite, be discrete and only do if it someone else had a better angle.
First of all, it is not necessary to ask the PU for permission to speak to the BU. Just call for time, wait until it is granted and go talk to the BU if that umpire made the call.
Let me add that if the only reason you are going to speak to the umpire is to ask him if his partner "had a better angle", don't bother. As was mentioned before, if you have a specific question on something you believed the umpire may not have seen (pulled foot, bobbled ball, swipe tag, etc.), by all means go and ask the umpire to check with his partner on that specific question.
Going onto the field only to question the judgement on the call should result in the coach being asked to return to the dugout.