Street

The first thing I would do is look at 'Twinmans' thread below "how to kit bash a twin. There is a lot of good info there to absorb. Next go to the thread "Triple Telemaster progress report" and scroll down to post #15 and 16. There you will find photos to a very simple method to make up underwing nacelles that are ideal for converting a high wing airplane and even easier if the wing is flat bottomed as yours appears to be.

Here are a few considerations you might want to give some thought: First is do sonsider giving both engines out thrust which become very beneficial when an engine quits. This can be simply done by attaching the nacelles to the wing with an out thrust angle or the firewalls themselves can be angled as normal.
Now some thought to power, assuming that airplane weighs around six and a half pounds as a single then .25 class engines will work fine. The fifteens may be a bit lethargic. I have done two twins like yours and .20's did just fine. If you stay with lighter engines the ship will work well with the small displacements but since bashing a twin usually involves an increase in weight the flying speed will be slightly higher. If you go to heavy large engines then weight becomes a problem and the best solution in that case is an increase in span by one rib
bay for better inflight handeling. For a first twin overpowering is not a good idea in my opinion.

Keep us posted on your thoughts and progress. You will find this kind of bash can be very rewarding.

John