My friend, if should be so lucky, and u have the area, money, and resources to have that wonderful option in the first place,- Then by all means, sock out a few extra duckets and go big... It will only come back to you in the form of bigger, happier, heavier trees... Of course, u could use mine, and probably a million others' method, and just do one, easy-to-deal-with 5 gal container per plant if u r going with two plants, as it's better to not let their roots become intertwined and lert them enjoy their own space, and and you'll definately see some nice returns... But for outdoor applications, I like to see big, ol' monster plants become all they wanna be, so I would even suggest, amending your soil to a less expensive "brand name" type, find a nice organic living soil recipe, which r usually very affordable compared to a "premixed" "brand name" type, and go with the nice 20-gallon size we used in Santa Cruz... You will be most pleased u did...