The stool came with the necessary tools (an allen wrench and a screwdriver). I did find that I needed to use a better phillips head screwdriver instead of the one that was provided to get the wood screws snug.
I would say NO my grandson is 19 months and it fits him. And even though it is very sturdy, I don't think it was designed for two. I should add that the bar at the top on the step up side does not work well. Makes it to hard for the child to get in and out. Grandson had a hard time so i took mine off.