Iím just a ploomber certainly not a mathematician but I would definitely say you will need to go with three of one brand and two of the other. You could also use a third brand for a spare but that might be confusing. Instead of having 2+3 you would have 2+2+1 in this case. One other option I suppose you could carry two spares and then you could have three of one brand and three of the other thereby; making it more symmetrical all the way around. Let us know what combination you go with. Boog.
I may have to grow old, but you can't make me grow up!