I would say between 1/8 and 1/4 of a cup per day each. Exactly how much depends on them. How active they are, if they gain or lose weight after the reduction of pellets, what pellets you give as well as just the differences from one rabbit to another.
If you start at one amount and that doesn't work, you can adjust it as needed until you get to the amount that works. Feeding rabbits is not an exact science, just guidelines based on many factors.
Since they have been getting unlimited pellets, you need to take them down slowly. Start by giving a measured amount that is close to what they eat now. Then reduce that by about 1 tablespoon a day until you get to the amount you want them at. You can do it a bit slower if needed if they are not adjusting well the to change.
If they are bonded, then make sure you use 2 bowls so each rabbit gets one. You might also want to put the bowls in different areas of the cage so one can't hog all the pellets.
I feed my rabbits twice a day, so you might want to consider doing that as well so you can space out the feedings.
