I'm with the others-two BSTs, or one bst and the other /bst. if you keep the mains and subs offset like that, you will have a great opportunity to level subs and have another job at full strength. just keep your bst main higher then the lvl of your other job when flipped. i'd suggest /rdm at the top-debuffs, cures, and *way* more fun then whm to level (imo). /whm is handy in beastwork only areas (raise and cures are top notch). If you are advanced enough to get it, i think /sch would be a dreamboat for a duo. your friend should start /dnc (for gil economy-there is a decent bit of overlap in some chm and general gear goodness for when you have to flip to /bst to lvl your sub).
i would highly recommend one/both of you learn to cook. use it to fill time waiting for the other to log on/get there. there is an incredible guide from ctownwoody in the wiki that can get you rolling fairly cheaply. not only will it get you some of the tools of the job dirt cheap, it will also help keep you in gil.
have fun^^