These need a conductor to touch the raindrop which ends up in a capacitor for the charge stealing to work. Any conductor you put on top of a solar panel will block the sunlight so it will decrease efficiency. The rain collectors equally need as much surface area as possible for the conductors with their underlying capacitor area close to them. It's more likely to be effieicent to have separate panels as a result. Also rain panels can face away from the sun and suffer no ineffiencies from that.
So the optimal strategy is solar facing the sun and rain on roof that is not.
Also make them harvest energy from the wind at the same time somehow.
“Come Rain or Come Shine”™