For purposes of this subdivision, a debt obligation on which there are two or more primary obligors shall be apportioned among such obligors, taking into account to the extent appropriate under all the facts and circumstances any choate or inchoate rights of contribution existing among such obligors with respect to the indebtedness. The term "agency or instrumentality", as used in this subdivision, does not include a possession of the United States or an agency or instrumentality of a possession.
26 C.F.R. §25.2511-3