I recommend roasting them separately (although at the same time) for one main reason: More surface area means more of the delicious brown crispy bits on the surface.
It will also take less time, probably, than if they are piled together or if you get an entire shoulder, but that is less important.
I disagree respectfully with MandoMando: pork butt cooked low and slow does not require brining in any way. It has sufficient fat and gelatin to be quite delectable after cooking.
I should add in terms of cooking time:
- If braising, 3-4 hours, probably
- If low roasting, 3-12 hours, depending on temperature, most likely in the 4-5 hour range at 250 since that is not as fast as braising at transferring heat energy to the food object
In either case, you will know it is done when it pulls easily. This is not a case where final internal temperature (you want about 180 F) is the goal; you also need time at temperature to covert the collagen in the pork to gelatin to give it that luxurious texture.