As they become more ready to be fixed, I don't think you'll have the liberty to choose to wait, based on what I've read from others. If one hits hormonal-instinct level first (which may most likely happen due to the age difference), this may cause fights immediately, so you wouldn't want them together anymore until both are fixed, for safety reasons.
That being said:
If they are only 12 weeks old and 8 weeks old, they most likely are not bonded, to the extent someone who is experienced with rabbits would feel comfortable saying that. They may be friendly to each other, but with them being so young, it's more likely for them to be friendly than not. The obstacle to overcome when bonding rabbits is the innate instinct to defend territory, assert dominance, and mate (essentially). None of these urges come about typically until the rabbit becomes more hormonal, which could potentially happen in a 12 week old and 8 week old at this point, but I'm doubtful it has, and if it has, bonding them in such a short time.
I'm biased towards always fixing both rabbits before letting them even attempt interaction (not interested in breeding), so I'm trying to be open minded, because I know unfixed pairs exist... But it's not very many, and I think they were generally under the guise that problems were expected as the rabbits grew up, and they fortunately did not. I don't think any responsible rabbit individual would feel comfortable claiming a 12 week old and 8 week old are bonded for certain. Baby bonds are very unstable because hormone development changes a lot in a rabbit.
