I don't know the answer for sure...but in humans calcium can hinder iron absorption if taken together for one example...and it may happen with other vitamins too...so rather than risk it I would give them separately.
Why would you want to dust with calcium d3 and a multi vitamin anyways? That really makes no sense. Some what of an overkill. Doesn't your Multi Vitamin already contain CALCIUM and D3. Maybe not but most in the reptile market do. Like reptivite, minerall indoor and others. Read your labels as simple as that. I think its obvious why you don't do them on the same day.
