Use wings that result in about 1.3 to 1.5 times the area of the single wing. Doing it that way means that the model will fly about the same for the same weight. The interference between the wings results in some minor losses in overall lifting efficiency. Adding a bit of area compensates for that. Also the closer you can keep the interwing gap to one wing chord or more the better. Again the interference grows the closer the wings are together in terms of chord length.

There's no real hard and fast rules and there's fine examples of oddities but if you stick to these guidlines your chances of an all around better model will be higher.