The umpire who made a call or ruling may ask for help if he/she wishes. No umpire may overrule another umpire's call. Specifically, no umpire shall criticize, seek to reverse or interfere with another umpire’s decision unless asked to do so by the umpire making it. The only time one umpire can overrule another umpire is when two umpires have made different decisions on one play. In that case, the umpire in chief (plate umpire) shall make the final decision.
LL Playing Rules: 9.02(b) (c)