Whoever has been admin longer gets 'Second in charge' rank. No permissions change but they get a higher rank. I sometimes higher up permissions though.
What I do is I do give my staff members a trial period with reduced permissions in order to see what they'd be like.
And I don't really have much of a 'Second in charge' rank, namely due to UKG having both a website, a forum and a few Game Servers, so there are an admin for each section so that we're able to share the workload.
If you are using phpbb NEVER give your other admins style editing permissions unless you really trust them. One edit to the style can stuff up the whole display of the forum and if people see that they won't bother coming back. This is mainly for promotion type forums though but it's one admin permission I would never give out.
I have trust issues with people. I've also never had another admin and usually have very few staff. I'm going to have to get over that if my forum is going to continue growing like it is. It's almost beginning to be too much.
Most of the time, any staff hierarchy amongst my staff is more implied than anything else. It's merely understood who outranks whom. Not entirely sure why, that's just the way things seem to work out for me.
i would say i would have other admins but on my other forums i use to have but forgot about i would pick out a few people and have a trial process for them over like 2 months