Can you use the reverse gears with new pinion out of both drives to make a good set

Yes you can. When its warranted, I often flip forward and reverse gears to get a longer life out of them.

Isnt that the same as swapping drives?

Not in my opinion. Because each gear came from different drive. My question referred to using them with new pinion. Switching drives stays together and has wear pattern. I'm using two reverse gears and new pinion.

Swapped reverse and drive gears because drive gear had very small chip on one tooth. Within an hour of running the reverse, now drive gear, blew three teeth off the base of gear.
Manufacturer flaw that was waiting to come out.

That's what scares me. Is the stress and being work hardened one way then forcing to go the other way. I might just take the new gears and replace the drives on the drives and since there's new pinions let the pattern wear for old gear. But what concerns me is will the new pinions take shape of the reverse gear and mess with the new gear in a bad way

Pinion still pushes on the gear the same way in the top or bottom ( Forward or Reverse ) position

like obnoxus stated the pinion gear pushes on the ring gears from the same side.it is common practice to switch drives from one side to the other every year to extend the life of a bravo drive.99 % of the time the drive is in foreword so the reverse gear is getting 1% wear.much easier and quicker to switch drives than to open it up and switch gears.the ring gear is the weak link,not the pinion gear.

So is it better too use a new pinion or the old one ?

I know they spin the same. And some have said just swap drives. So I will explain better. I have both drives apart. I have one new gear set which full price is a ripoff I got builder price from a buddy at formula for 1298.00 a set. But I got the new pinion and thought I would run reverse gears with new pinion or just get another set of gears.