And if you're going to build in sheet foam material instead of solid EPS foam, many successful models have been made by enlarging paper models with your computer and printer, such as the ones at Fiddler's Green. There are many historical biplanes there.

Generally, to convert them, you have to butt instead of lap the cut-out skin pieces, and add some formers internally for stiffness. The wings can be sheet style undercamber airfoils, or wire cut foam -- undercambered single sheet style wings would tend to make a lighter and easier build for a biplane, but you could do a double surface or solid core if you really wanted.

Course if you want to come up with your own design, rather than a scale bipe, any of the other methods should get you there