Small speed and manouver penalty, none of them can attack.
Not sure if they should be able to block. On one hand, they should have some way to defend themselves, but on the other, we don't want 2 shielders 1 plated charger just tanking around.
The best would probably be that the rider can block and neither of them can attack. You still got some defense, but you can't do tag team turtling.