If a person was willing to spend the money and had reasons to desire more options (eg a health issue, etc) one could carry a PLB and an InReach Mini. I've thought about it.
One certainly could.
For assorted technical reasons, including more watts, and redundant means of finding location (Doppler as well as GPS), the PLB
might get an SOS out
in a few specific situations where an InReach might fail. However, this a probably a mostly theoretical advantage. In the real world, the InReach seems to do quite well.
If I were looking for a backup to an InReach another option to consider would be a Sat Phone. However, this would be significantly more expensive, and would involve carrying somewhat more weight and bulk.