The Garmin Descent Mk1 is based on the Garmin Fenix 5X - or more accurately, it IS the Fenix 5X with dive capabilities added.
It is likely the Mk2 will be basically a Fenix 6X.
There would be no value at all in having two Garmin devices. The value of the Garmin is the fitness/sports/everyday features. You can only practically where one watch for everyday use and fitness activities.
If you wanted redundancy, you'd be better off with a Garmin and a Shearwater. Even the Perdix would be perfect for that. You could set the gas factors the same.