Roles
Take 2 minutes at the beginning to introduce two roles: "pilot" and "co-pilot." You could say: Just like in an airplane, the pilots are head honcho, but let's be honest, it's the co-pilots who end up doing most of the work.
The pilot is the one who gets to make most of the decisions... "Make it blue..." "Make it smaller..." "Click here or there..."
The co-pilot is the one who is actually working the controls. They're steering the plane - or in this case, working the mouse and keyboard.
Occasionally, pilots need input from their co-pilot. "That's a weird color for this thing," a co-pilot might say. Or, "hey, how about we try it this way..." They still have a say, but it's the pilot who has the final say.
So the two coders will spend their time with the co-pilot working the mouse and keyboard, but the pilot telling them what to do.
Then switch roles half way through, so the co-pilot becomes the pilot, and vice versa.
Who Should Share?
You'll want to try to have kids share who are roughly in the same place. Even if they're not in exact the same spot, it'll help to have them generally in the same place/at the same level of experience.
I'd suggest you take 5 minutes at the end of each session for the kid who wasn't signed in all day to log into his/her account so you can sign them off to the point they were at as a team. This will keep every kid's individual account at the appropriate point.