Well... they both can drink from the bowl, despite not being very taken with it, so i'd say you can be quite tough. Mother cats would be letting them go for substantial amounts of time without nursing at this stage.
Give them half their usual morning amount tomorrow and try leaving them until lunchtime and see if they get desperate enough to at least drink from the bowl, preferably munch some food.
There's nothing wrong with them having a couple of bottle feeds a day at this age, they're still tiny and like human babies, nursing from bottle or teat provides more than just milk, it's comfort and security too, it would be cruel to take it away altogether. But they are getting very big to be surviving on only milk!