I'll chime in.
They should definitely be kept separate for several reasons (like previously mentioned).
- First of all, the size difference. One is tiny, one is giant. Mishaps could happen and the little one get hurt or killed just by accident.
- Next, the size difference. I don't know what sex they are, but if they are a doe and buck and they breed, it could be a big problem if the Netherland is the doe.
- Also, if they are both females, there could be blood. Rabbits are territorial. In cages it's never good to raise does together, especially if they are not siblings.
There are probably many reasons. But I'd highly recommend different living quarters for them unless you want to gamble on their lives.
Good luck and congrats!