I'm not surprised you have a headache,
There's a great deal of new information to understand. Take it slowly a bit at a time.
With 2 bunnies it's OK to have them on the same diet.
Bunnies can actually live healthily to a grand old age without pellet feed at all.
House bunnies need the vit D3 in an eggcupful of pellets daily & to be in outdoor light for an hour a day.
When pellet feed is reduced to an eggcupful, you need to check they're not losing weight, weighing at same time of day about every week. Weight
does fluctuate a bit, (poops inside) which is why we weigh at the same time in the middle of their poop cycle.
Of foods which increase fluid intake but aren't too rich, a piece of cucumber is good.
Dry dandelions are fairly high in calcium & to be avoided but fresh dandelions have a lot of fluid, & is A diuretic so make bunny passes more urine. Most sludge bunnies benefit from fresh dandelion leaves & some fresh herbs fed while still wet after a rinse. But that is for next growing season. winter is a difficult time.
My own bunny's main problem is tummy (cow pile syndrome/dysautonomia). (He can't drink enough because his tummy is slow - a throughput problem - not enough room to eat & drink, & is totally intolerant of human veg)
A handfull of hawthorn leaves does wonders to speed up the gut, & blackberry leaves are even better. Both can be foraged in towns & there are loads in the country but the leaves are falling now so you'll need to get some in fast for winter.
ETA It's really painful for humans to pass "sludge" we call it gravel in humans. I expect it's the same for bunnies. Pain slows down the gut too.
what sort of tummy issues does your bunny have with veg?