If I may list some of the variables in play,
#1 Two baptized, practicing Catholics,
a. married "in the Church" with the priest's blessing - a valid and binding marriage
b. married "outside the Church" with non-Catholic witnesses - not a valid marriage
#2 Two baptized, non-practicing Catholics (not rejecting the Faith, but not practicing it),
a. not able to marry "in the Church" without returning to a practicing status, so
b. married "outside the Church" - would their marriage be valid and binding?
#3 Two baptized, practicing non-Catholics (Protestants),
a. not able to marry "in the Church" without becoming becoming Catholic first, so
b. married "outside the Church" in their Protest religion, a valid and binding marriage
Did the ability to marry "in the Church" change after the Second Vatican Council?
Is marriage the modern, novus ordo, post Vatican II religion #1, #2, or #3 above?
Is marriage in the SSPX #1 above?
Is marriage with an independent Catholic priest #1 above?
Is marriage with a CMRI priest #1 above?
What if (unaware of SSPX or CMRI) the two just went off and exchanged vows privately with two witnesses?
One further variable, what if two baptized, traditionally practicing Catholics (say for example SSPX or CMRI) return to the modern Catholic practice; Was their marriage in either of these traditionally oriented organizations "in the Church"?