Concur with all.....a BGO does not make or break an application. Feel free to work with both...sometimes its nice to have two perspectives. I've mentored candidates without being the "official" BGO several times. Because of my job, I know a lot of people in community, and I try to avoid the appearance of conflicts of interest (and preserve relationships) by having the sons and daughters of friends and clients assigned to another BGO.
If you do "consult" with multiple BGO's, I would recommend being open with both, and let them know that you have been talking to the other. I've never heard of any problems among BGO's, but its human nature to be territorial, and while I hope it wouldn't happen, its possible that the "official" BGO might take offense, or think he /she is being circumvented. As a BGO, I avoid this by getting in touch with my counterpart and telling him/her about my relationship with the candidates family.