Unlike cats and dogs, rabbits aren't usually just routinely fixed at around 8 weeks old. Usually for males it's when their plums drop at around 3-4 months at the earliest, and for females it's usually 4-6 months. But call around to the rabbit vets around you, and ask when's the earliest they fix each *** of rabbit, so you can start making plans and vet appointments.
It's not impossible for your rabbits to be able to bond, it's just better to do it the other way and let them pick the companion, since there are most definitely some rabbits that seem to hate each other and are unbondable. I had a pair like that, two females. I tried for months, all different methods that were possible, and they just never would get along.
For now, if there's a chance at all to be able to keep your current rabbit and the one you're getting together before they're both fixed, you would have to first confirm the *** of your current rabbit, then the *** of your new rabbit. If your current rabbit is a female and your new rabbit is a male, when you get him if he's less than 10 weeks old, then you may be able to keep them together for a week or two before needing to separate, as long as neither shows signs of aggression or wanting to hump the other rabbit. Males usually start to become hormonal sometime after 10 weeks and females sometime after 12 weeks. So both would need to be under those ages, particularly the male.
Now if you end up having two females(and this is confirmed or you could end up with babies or two males fighting), and you put them together with no signs of aggression or humping, then it's sometimes possible they will continue to get along and can be kept together until they are 4 months old and old enough to get spayed. Though you always have to very closely monitor their relationship for signs they would need to be separated until their spay instead. You don't want to risk it because if you get two pretty dominant females and one starts pestering the other, it can irritate that other rabbit to the point a fight could break out, resulting in potentially very serious injuries.
If you have a male and female, if you can find a vet that will neuter at less then 12 weeks old, you may be able to bond them pretty quick. If you have a male/male combo. You'll need to keep them separate from the start, get both neutered, usually wait at least 4 weeks for hormones to fade, then attempt to bond.
If you don't want to risk anything, just keep both rabbits separate regardless of *** until both are fixed, then attempt bonding at the appropriate time(usually 4-8 weeks post neuter/spay). Bonding can still work, it's just far more complicated than getting an already fixed companion and doing dates to find the right match.