The problem is, that if only one company manufactures a vaccine (as I believe is the case with mmr) then if they decide it is cost effective for them to ONLY produce combined vaccine, then they will stop manufacturing the individual vaccinations, making it impossible to obtain the vaccines. I'm not sure if licencing for animal vaccinations is the same for humans, but I tried for years to get my gp to obtain individual vaccinations even when I offered to pay for them, and they wouldn't do it. Then all the private clinics in London stopped offering single vaccines cos they could no longer get them.
I agree with Jane that having two vaccines together could adversely compromise the immune system, especially in bunnies with already compromised immune systems.