61 Pa. Code § 109.2 – Husband and wife.

(a) Both nonresidents subject to tax. If separate returns are filed by a husband and wife both of whom are nonresidents and subject to tax, then their income from sources within this Commonwealth shall also be separately determined.
(b) Both nonresidents but only one subject to tax. If only one nonresident spouse is subject to tax, then only the income of that spouse from sources in this Commonwealth shall be subject to tax.
(c) One spouse is a nonresident. The following requirements apply:
1. (1) Separate returns. If either husband or wife is a nonresident and the other a resident, they shall determine their tax based on their own separate incomes and their tax liabilities shall be separate. Reference should be made to § 117.1 (relating to general requirements of a return).
2. (2) Joint returns. If both spouses elect to file a joint income tax return, then their income shall be determined as if they were residents and their tax liability shall be joint and several.