WAV "... on keyboards I use both hands for just the top line"
Try playing a drone with one hand. Or better, get an Indian harmonium. These usually have drones - up to five - and you pick the two (tonic/dominant) for the key you want. One hand pumps the bellows, while the other plays the melody. No need to do more, although you can throw in the odd fifth or octave, or be more adventurous if you want. Drones give a basic harmony, and playing or singing against drones is a long-standing tradition in popular music throughout Europe and the Middle East; bagpipes, hurdy gurdies, zithers of the langeleik/epinette/Appalachian dulcimer type, and fiddles have used drones since the Middle Ages and earlier. England has not been immune.....
John Kelly.