Im a vet nurse and we always prescribed twice daily dosing as rabbits metabolise metacam more quickly than cats and dogs so twice daily dosing is said to be more effective for the rabbit. HOWEVER it does depend on the dose you are giving. You CANNOT give a once daily dose, twice daily as you will over dose the rabbit (in cats and dogs, overdosing metacam can lead to kidney dysfunction and gastric ulceration. There have been no testing specifically in rabbits as the drug is not licensed for them, so we cant be sure of long term effects or overdosing effects)
I would recommend you speak to your vet re dosing twice daily and the dose to be given. We use the dose receommended by the vets who have spoken at the RWF rabbit conference. That dose is 0.1-0.2mls per kg twice daily.
Hope that helps. I would like to stress that you should discuss dosing with your vet before you change what he/she has recommended.