I was thinking they get caught/seperated because William and Abraxas get stopped by some local knights who ask what their business is. They can't answer since they don't know how to copy a Younisian accent, so they either try to run away or William leads them in a fight that doesn't end well.

Maybe William and Abraxas get captured as prisoners for trespassing while Crow, Penelope, and Hartley escape or get set free since they sound native