I agree. Two ferrets would be better. It would be better in the field as well if you have rabbiting-trips for almost the whole day. In my experience, since ferrets usualy sleep about 20 hours a day, one ferret will not be working that fast and good when you've been working it for a few hours.
So when I go out ferreting for a day instead of for just a few hours, two ferrets would be my choice.