You should definitely play with both of them all the time. They deserve it and you deserve it. The one that's not as sensitive might also get more sensitive with further play and the increased size that comes from getting used and toughened up.
I'm right handed so when I'm laying next to another bate addict pig, I like him to be on my left. That way I can put my left arm around him, pull him close, and let him worship and nurse and chew on my left nip, while my right arm/hand is free to stroke my cock. I never noticed much difference in size between them, but that left one that got used rough so much definitely became more sensitive. I definitely noticed that! When I got my nips pierced, the piercer told me my left one was 5/8" across and the right one was 1/2" across. It wasn't visually noticeable, even to me, but I started letting guys go nuts on my right nip as much as possible. I think they're equally sensitive now, and I just had to go get a wider piece of jewelry for the right nip cuz it's now 5/8" too.