Thanks, everybody. I ended cooking it close to 5 hrs, and as pierino suggested one piece cooked faster than the other. But since I shredded the pork (I was making Cochinita Pibil, a yucatecan dish) it was really good, once the pieces got mixed together.

Closer to the time for larger roast. But be sure to check the internal temperature. Odds are one piece will cook faster than the other if they are different sizes.

I would treat it like you would any piece of pork shoulder/butt, that you would use for a braise/slow cook/pulled pork. Cook it low and slow until it is pull-apart tender. Since both pieces are about the same size, they should break-down-tender at about the same time. This will just cook faster than it would if you had a 7lb. piece. You get to eat it sooner, which will be nice, since you will be hungry from the house smelling amazing for quite a while.

I would think you'd be calculating a roasting time closer to 3.5 pounds, but you could just take the guess work out and tie the pieces together with kitchen twine and roast as usual.