I believe a twin engine aircraft, with 2-axis control (i.e. Rudder and Elevator), under normal circumstances is flyable. But is it safe at all? If one of the engine dies, will the rudder alone be sufficient to counter the consequent roll/yaw?
For an ultralight with rudder only roll control, which achieves lateral control with a lot of dihedral to create a really strong roll/yaw couple, the issue would be how much reserve rudder authority is there once enough has been applied to counteract asymmetric thrust. That would determine how much roll rate you could still achieve trying to yaw toward the dead engine. If the engines are reasonably close together, it would probably be flyable but you would have a lot less turning authority into the dead engine than away from it. In the end, you would have to go try it to see what happens, or get someone to do it with an RC model version.
I think your problem is more to do with simply single engine performance. If one quits you are now at 50% power. Using two engines doubles the chance of engine failure, and if you can't climb decently on a single engine, you're still in a glider after a failure, just one with a flatter glide. IIRC, the Lazair twin was just able to maintain level flight on one engine at lower altitudes, and it was pretty clean as ultralights go. The Bloop being a lot draggier, you would probably need more total HP that it uses as a single, to have decent reserve power when one fails.