Nooo... a BD-50... I should break my piggy bank now! (^^,)
You know I'd like to have two BD-25 instead one BD-50, but it's your project and it's up to you; so, if you will release in on a BD-50 anyway, just fill the disc with every possible audio tracks!!!
Hint: never done a BD-50 before, but could be useful to do the main features in two .m2ts files, the first to be a little smaller than one BD layer; this should avoid eventual lag when the laser passes from a layer to the other - a thing sometimes noticeable on DVD9, don't know if it is the same with BD-50, but I'd prefer to stay on the safe side anyway!