Slyvan is right on the money about these two heaters being plumbed in series with same length piping.
This solar system sounds like a heat exchanger set up that we use to capture tempered reverse osmosis reject water. We use reject water to preheat incoming city water before the heaters.
Use thermometers in separable brass wells placed pre and post of your solar tank this will enable you to calculate your temperature differential. The higher the differential the better the efficiency.
Respectfully David F. Walling