Hey Annie, and everyone.
I'm working on the twin rudder arrangement. It has required some thinking but I now have a plan. It will be some time before it is drawn up though. In principle there will be twin rudders canting outwards so that seen from behind they will be more or less perpendicular to the edge between the transom and the bottom of the hull. Each rudder will be attached to a skeg that extends form the bottom of the hull and half way up the transom. The rudder stocks will be attached to the skegs and come up through the transom but will be hidden under the aft bench in the cockpit, the same space that will hide the outboard well. There will be one tiller and a tie rod - ball joint linkage between the tiller and the rudder stocks. This picture (but with skegs) will give you the general idea.
Although the canting rudders will make for a better bite while heeled I would still like to make them swinging to provide enough area. Each rudder will have to have around 0.4 square meters of area to match the sail area. Making them 0.4 meters without going deep will make them very long, around .65 meters. That will make for a lot of rudder behind the boat that you can bang up against things. It will also make for heavy steering. Look at a lot of the older cat boat designs, like this one that will be quite heavy on the helm and difficult to control when heeling.
Now... The version of Sib-Lim with longer non swinging rudders would still behave better than the cat boats of old thanks to the canting twin rudders as compared to the single non canting rudders often seen on those boats, so this is still a viable option if you wish to avoid the complications of swinging rudders.