When I adopted my male I had an agreement with the rescue center that I could bring him back if the fighting was too intense/they wouldn't bond. It's an unpleasant reality to some people to talk about but for me it was needed. I too did not want to house two buns seperate forever either. No one wants to bring a pet home thinking you may "return him/her" but in this case I view it as being realistic and responsible if it's something you are honest about upfront.
With that being said there still is no way to guarantee bonds won't break along the way. However, I think there are things to do to help odds work in your favor.
Both buns should be fixed (sounds like you know that
) M/f bonds generally are easier/are stronger than same sex bonds.
I also think (these are just my own theories after having a bonded pair) that consistency with rabbits help bonds stay intact. In other words, no changing homes, locations, new pets in and out etc. Also, plenty of human interaction. I would never just lock a bonded pair in a cage day in and day out for the risk for them getting stir crazy and fighting may occur.
Also, always have two seperate cages in case. And when you bring the new bun home, don't feel the need to rush things. Sometimes bonds happen in a few days, sometimes it's months.
And, research and understand bonding and have a realistic expectation of bonding. Even if a bonded pair may experience trouble that doesn't mean they are broke up forever. You just may need to rebond down the road.
I think if you have a good understanding of all these issues, have altered buns, go with a m/f pair, then your success rate is very high.
I've had a very enjoyable and highy successful experience in bonding. Some on here may call it lucky, but I also was EXTREMELY worried about the decision to get a second bun and I researched a lot and took it very seriousy so maybe that's why the success as well