As a start you could try this;
- Pick some models that do well with the engine as a single that have performance that you'd like to emulate in your twin.
- Take the average wing area of those designs and double it.
- Work on the structure so that the weight comes in at double the average for the singles or within 15 to 20% max over double. This would be OK since the added size increases the Reynolds Numbers and thus the allowable wing loading for good performance goes up. But you'll lose climb performance if you let it get too far.
- Add a spot to put the two engines on this new "double area/double weight" design.
Yer done. Just build and enjoy.
This is the method used to build the DH Mosquito of WWII. They took a look at the Spitfire and wanted a longer range twin to mimic the Spitfire's performance. This led to two of the Spits Merlin engines mounted on a plane with twice the Spit's wing area and *****g out at double the weight with double the wetted skin area. The results spoke for themselves.