Scieszka and Mac Barnett were at SCBWI-LA this August and did a workshop focused exclusively on how they created BATTLE BUNNY (which included a pretty hilarious read-aloud, with the two of them simultaneously reading the two storylines). Yes, they collaborated directly with Matthew Myers. Don't know if they'll do a workshop with Myers, too, someday, but if you catch another workshop with Scieszka and Barnett, they did talk about the collaborative process quite a bit.