I would say it depends on two things: Thing 1: Are you housing livestock in your sump? If so, it makes more sense to have a redundant pH probe in sump.
Thing 2: as a corollary to thing 1, if your CaRx dumps into the influent side of the sump, passing OVER said livestock, it again makes sense to have a redundant pH probe.
Realistically, you are backing up a backup, to save the media in your CaRx, because your DT pH probe *should* catch a dangerous drop in pH. Seems like the economics don't support the cost of an extra pH probe + calibration vs the cost of an entire container of media.