One vehicle or two?
If you both want to drive, take one each. You are in the same guided group on the same route, so you ride together anyway — you just each have your own machine and your own line to hold.
If only one of you wants to drive, share. The half-price passenger rule makes that the cheaper option by a clear margin, and it means neither of you spends the ride alone in a convoy.
Why most couples take a buggy
Because a buggy puts two people in the same vehicle rather than one behind the other, which is a different kind of afternoon. You can talk. You both see the same thing at the same moment. And a nervous passenger sits beside a driver instead of holding on behind one.
Sharing a quad works too and follows the same half-price rule. It is simply a more single-file experience.
If one of you has never driven anything
That is the common case, not the exception. The briefing and the test drive are built for it, and if the guides decide on the day that someone should not drive, they ride the same route as a passenger at half price.
Which means you can book two vehicles hoping you will both drive, and still have a good day if only one of you ends up doing it.
The practical bits nobody mentions
You will both be dusty. Dark clothes, closed shoes, sunglasses — and if you have plans afterwards, a change of clothes in the bag is a better idea than it sounds.
Book both of you in one reservation rather than two: it is one confirmation, one pickup and one conversation if anything needs changing.