There are actually a couple of interesting effects at play here which you should consider when designing such an arrangement.
The first is that two rudders either side of a prop like this actually creates an effective nozzle, deflecting the vast majority of the wash exactly where you want it. Bear in mind though where the pivoting point for the rudders are hence are they a balanced, semi-balanced or unbalanced rudder. The ones you show above are unbalanced.
Then if you think about the cross sectional area of the space perpendicularly across the rudders as they turn it actually gets smaller. This can create an eductor effect and will lead to an accelleration of the water out of the rudders and therefore an effective turning force. The disadvantage is when turning high rudder angles at high speed as the reduction in cross sectional area can effectively throttle in the wash and stall the flow over the rudder. An advantage is that you get effective steering at slow speeds!!
What would be ideal with this arrangement would be a mixer that reduced the rudder angle depending on the power to the motors. The faster they go the more the maximum rudder angle is reduced, now there's something to think about!!