Replacing the pair together keeps both strings in a course at a similar point in their life. Match the material to the rest of your set. Check how your instrument anchors its strings before ordering.
In the 4th, 5th and 6th courses, the two strings are tuned an octave apart, so the higher one is a lighter gauge. In the 1st, 2nd and 3rd courses, both strings share the same pitch and gauge.
Bajo sexto strings aren't a drop-in replacement for a standard 12-string guitar set. They're gauged and paired for a different tuning, course layout and instrument. Check the specs, and consult the manufacturer or a qualified luthier when in doubt.