Hi sailorsgal!
I know a number of people who have done shared teaching or hiring a tutor as a supplement to their child's education at home, but no one who has done it more than a day or so per week in total.
You may be envisioning that the children need 30 hours of education a week and feel that it would be hard for parents to provide that, and want to share the burden? The thing is, home ed is not actually that hard and most people don't do anywhere near the number of hours that schools do, because the learning is so efficient. Home ed is easier than you think.
Coordinating with other families is the part that can sometimes be an effort, though that doesn't mean it can't be worthwhile educationally. (I was just talking to another HE mum in the park yesterday about her experiences of this! She'd taken a six-month break from her group recently because her family was dealing with traumatic personal problems and she didn't feel up to the shared enterprise. It was easier to home ed alone during this time. Now she feels ready to join in again.)
Why don't you start by going along to a local HE group and chatting to other parents to see what home education is like? If I were thinking of an arrangement like the one you describe, I think I would first start by home educating solo for six months or so in order to get stuck in without the complication of working with other children and families. That way you could get a feel for how your children learn best and what interests them. You'll discover what, if anything, they are missing out on which might be usefully supplemented with a group.
You also might have time to meet more local HE families and see whether any of them already have an appealing group in place which you could join. It might be that the few friends you've already spoken to aren't the ones with whom you and your children have the most in common, and you'd rather join different families instead. Or you may do a pick-and-mix approach, in which you share science lessons with one group and do art with another. That's what we do for maximum flexibility. The kids with whom my dd does maths are not interested in the same sport as she is, so she does sport with different children.
Good luck! I'm sure you can figure out a suitable arrangement. You don't have to have it all in place before you take your children out of school.