Engine is a 17.5 briggs@stratton twin model#42a707 only runs on one cylinder

Hi,
First check that both sides have compression(ie that the pistons are moving.) It is not unusal for that engine to break a rod on one side. Otherwise if not a fouled plug, probably half of the high tension side of the armature(coil) is bad. That engine fires off one coil and fires both sides every time the pistons rise even though it only makes power every other time. Unfortunately my guess is the first.
Peace,
Clarke