Of course you can do that. The objective is to apply even rotational speed and torque to both motors on the same end of the truck (or all four if the need arises, though this would be rare) such that the traction control doesn't have to do the thinking to find grip.I don't disagree that a locker would be an improvement. However, you aren't going to get the the same behavior without that physical connection.
Conceptually Rivian may be able to switch from torque control to speed control with the motors or something, but I suspect that they have reasons not to do that.
The disadvantage of this, specific to the Rivian, is that each motor has a finite amount of power and torque available for each wheel. Being that the Rivian is not a lightweight vehicle by any stretch of the imagination, if the amount of power in each motor that has grip isn't enough to get the truck to go...you're stuck. This is where low range is helpful.
At the end of the day, I wouldn't take my R1T anywhere close to the places I took my old Jeep. It's just not made for that and that's ok as it does a whole host of other things better. With a Jeep (or similar) the live axles help to keep the wheels on the ground to prevent a need for lockers to begin with. In all my time off roading I can probably count on one hand the number of times I've used the rear locker and I never used the front.