Im weighted to be neutrally buoyant near the surface, with no air in the BC, for whichever is the worst case of either main tank near empty with the pony detached or with both the main tank and pony near empty.
Since the pony with regulator is about 2 pound negative when empty, the worst case is with the pony detached. So I ignore the pony in regards to weighting. As to trim, I like to shift one pound from the left (where the pony is stage slung) to the right. The pony is about 5 pounds negative when full. With the pony full and attached Im skewed three pounds to the left, and with the pony detached, two pounds to the right.
Hope this helps.
Mike