Hi Rina,
I have three school-aged kids and my 3 yo will be doing preschool this year. The group subjects--science, history, etc.--are the most challenging for us because I am constantly having to tell them to stop talking to each other, looking at each other :laugh:, etc. Maybe you already have a good handle on this with your boys, but adding two more kids from another family could certainly change that. Will the other mom stay around?
We have done co-ops that meet once/week for up to three subjects, and at least one other mom is there to help. I have loved our co-ops for their fellowship for mom and kids as well as the academics. But I know I would find it difficult to do the same co-op multiple times a week, or every day it seems like? That's just me! I would also be hesitant about being responsible for another child's reading and math instruction. These are just too important.
If you have become close, have the same values and academic goals, and go into it talking about expectations of behavior, rigor, etc., I suppose it could work. And I hope you do work out something great for both of you.