Spaying can be done earlier, but it depends on your particular vet as to what weight they like females to be before they are willing to spay them, some don’t like to do it when the rabbits are still quite small. So depending on which vet you’re planning on using, your best bet is to ring them to ask, then given how much Hollands are inclined to weigh at a certain age, you can guess roughly what age you can spay them based on that.
They can be bonded before spaying, and it has been done, but it certainly does pose risks, especially if the vet wants to wait until 6 months of age. You will probably need to separate them around 4 months, depending on when things start to kick in, but it really does vary a bit. You just have to be watching them to make sure you separate them BEFORE they start fighting. There is always a very slim chance that they will continue to get along, but it’s not something to count on. After surgery you’ll need to keep them separated for at least 3-4 weeks as it can take that long for hormones to leave their system. If you get them spayed before they start showing hormonal behaviours you might be ok to start introductions from 1-2 weeks, but it still has its risks while they still have hormones running around inside.
Read up lots on bonding, it can go like a dream with some rabbits, and be a 6 month long process for others. At the end of the day, you need to remember that there is no guarantee that your two rabbits will bond, even when spayed, some rabbits simply don’t get along. When you do start bonding, just remember to start slowly, jumping ahead quickly leads to problems.