I had this concern up until pretty recently actually. I have two rabbits, one being a Netherland Dwarf (<3lbs) and the other is an American Sable (5-6lbs). I had my ND first, and he is a very slow breather. When I adopted my AS, I was concerned about her breathing. It was audible and super fast, even when she was laying down; however, over time (I’ve had her for 8mo now), I have come to realize that’s her baseline breathing rate.

I just counted breathes for each rabbit now:

ND – 9 breaths in 10 seconds — 54/min
AS – 30 breaths in 10 seconds – 180/min << this is very high, but she’s been examined so we just constantly monitor her

… I think as long as it is the “norm” for the rabbit and it sounds like clear breathing, you’re generally OK. Also consider the context: is it very hot? Did they just sprint around? Are they nervous at all? Interestingly, I submitted a question to Oxbow’s main vet re: rabbit size and their breathing. It is usually expected that larger rabbits have lower breathing rates, so mine are the opposite of that. So you want to weight what you see online with your personal observations and understanding of your rabbits’ health.

At least from my perception, a lot of owners don’t really measure breaths/minute. Breathing concerns tend to come when you can audibly hear the breathing, like something is clogging up the nostrils or it sounds very strained/taxed. This was the case for my AS when we first got her, but it has now become completely silent (hopefully a sign she’s getting more physically fit and healthy).

On a side note, are these two NZ’s being kept together? Were you told what sex they are? If they are together, in the next couple weeks, their hormones will kick in and they may very suddenly not get along. Also, misgendering rabbits is very common, so be aware that you may have a M/F pairing who will mate if kept together when hormones begin developing.