If they are just touching noses, I see no threat. They might just be smelling each other. If you see them lunging, nipping, or stomping then I would be wary of that.

I think creating a good trusting relationship(s) with the bunnies first is a good idea. I have no academic references to this, it is just my own theory but I think that if your bunny trusts you and sees you as a safe person, then he will be a bit more at ease when meeting a new rabbit because he knows you wouldn’t put him in a dangerous environment / situation. This can help with the bonding process. As you said, the bunnies are nervous of you, so when meeting each other and you there supervising they will have double the stress!

The best way to bond with bunny is just sit down on the floor in his area and pay no attention to him, read a book or something. It may take minutes, it may take hours but eventually his curiosity will get the better of him and he’ll come to investigate you. Let this build up until he comfortably lets you stroke him, walk towards him, move suddenly etc etc… My bunnies will now allow me to step over them when they are bunny-flopping on the middle of the floor! There are some key signs that he is relaxed around you (this can also be used as indicators or relaxation when bonding with another bunny). He will groom himself, he will lie down (there are a few different lie down positions to suit level of relaxation), his ears will relax and if he is reaaaaaallllyyyyyyy relaxed and happy, he will purr.

Once you have achieved this, there are some things you can do to prepare the bunnies for their meetings. You can switch their belongings, such as litter tray, food bowl and toys (not all at once!) which will help encourage sharing. Following that, switch their living area for a day at a time. Then, have the meet. Even though they have been used to each other previously, they have always been kept separate and had their own toys and trays which will make them territorial over them. The best thing I did before bonding my two was read. Read everything you can about bonding to help you prepare for various scenarios, because every bonding situation is different and needs a tailored approach!

Good luck