Hai Maharana, if you are using two props to your craft,its need not to use any rudders to it.you will get a nice 360 degree turn from a stand still ith two screws , that turns in opposite direction.i was making water based crafts from my child hood.you told us that you are using gear boxes to the drive assy.you can simply control your craft by using two micro switches for each motors, with using your two sticks simultaneously to foreard, or reverse.to get a turn , for eg to port side, you can simply pull the left stick towards you and the right to out from you..so your craft will turn instantly within its length.you can see here a model that i just finished with twin rotors.

maharana..dont worry..you need not to use any rudder for your craft if you have to screws..

i have compleated many crafts that uses two ,three , and four props..actually i made this craft for my sister, Rathi, who cries for it...i used a toy R/C for it based on 27 mhz.

by using its directly proportion motor controlls it works nicely at 7.2 volts.the two motors was a toy motors, so i reworked on its armature to withstand high temprature during operation.it flies over water...

you can see a craft here lengths up to 1.5 meter, all driven by three toy motors..with toy R/C..again it was finished for my younger sister rathi..it also works well..for hours...

you can accomplish this control by using two methods..first one ,can use single pole double throw micro switch,and for higher amps, can use same type switches made for washing machines.or can use relays for the same purpose. the second one is that you can simply use the throttle stick for foreward and reverse with speed controller, while the second stick, for left and right..if you are interested to do that i will give you all circuits here with plessure..remember..using a rudder assy to a twin prop craft is a waste...

Well it is nice of you to show interest in this boat. 
It is a more static model than RC,
and me a hobbyist than a professional. 

Maharana Sahab, that is humility. Bravo! If I could ever build a boat like the Mistral, my feet wouldn't touch the ground - I would consider myself to be a Champion.