(a) Separate Return. If the income of husband or wife who are both nonresidents of this Commonwealth and are subject to tax under this article is determined on a separately filed return, their incomes from sources within this Commonwealth shall be separately determined.

(b) One Spouse a Nonresident. If either husband or wife is a nonresident and the other a resident, separate taxes shall be determined on their separate incomes on such forms as the department shall prescribe, unless both elect to determine their joint income as if both were residents, in which event their tax liabilities shall be joint and several.