For the areas and the nationals - you haven't a hope (the people above are correct!)
For the majority of other contests I have been to, (including some larger ones like Pontins) a band may borrow a set number of players from the same section or below. They usually require the players registry card and/or a letter from the bands secretary that the player is registered with.
It is best to check each contest individually though as some do use the same rules as the area contests, and others allow different numbers of borrowed players that one band can take on stage.
As for playing with two bands at the same contest, this is permitted at contests where bands can borrow. (I have done!) The only thing to remember is that should both bands have a rehearsal on the day of the contest, you will not be able to attend one of them, and that the band you play with second will have you either 'warmed up' or 'tired out' depending on the first contest piece you played! ;-)
IMPO, I don't like guesting with another band before playing with my own band, which can potentially happen if they are in the same section, but that's just me!
Hope that helps