I have bashed several planes into twins.
Here are my two cents.
Example one. The Ultra Sport 1000. It had a St 3000 rated at approx 3.5 HP and an 18" prop. Did not have unlimited vertical but flew well.
It now has two ST 90's with 14x6 props......however....each engine is rated at 2.5 HP and as stated above, much larger disc area. The plane will fly straight up and much faster. Yes, I did reinforce the wings and control surfaces.
The wing loading will almost always go up on multi engine planes due to the additional controls, nacelles, tanks, ect.
Initially the plane flew "Heavy", but with so much power, was only a problem at low speeds.
I added a total of 6" to the wing span and wing plates to increase lift.
Example 2. I also bashed an ugly stick 60 from one ST 90 to two ST two forties. This did not fly well as the wing loading sky rocketed. Good power, but engine out meant it was coming down hard.
Changed to two ST 60's, added 8" of wing span, and wing plates. It then became a real blast to fly.

From these experiences maybe we can get a few ideas.
1. A single to twin conversion will increase the wing loading and that must be considered.
2. Don't underestimate the need to address the increased structural and control surface stress.
3. As a general rule, using two engines, one half (Or in your case one third) the original, will also increase the available horsepower and disc area. All of this will increase the performance of the plane far beyond the original. This is particularly true if the new plane's wing loading is similar.
4. Don't forget that longer wings also add drag that must be over come, to at least some degree, with the new design.
I will now get off my squeaky soap box.
Good Luck