Multiple DC Input to Single DC Output
Connecting them in parallel may be a problem since the lower voltage cell will drain some current from the higher one. I don't know if this will damage the cell, but it doesn't seem healthy anyway.

Basically I can think about two options: a DC converter for each cell to equalize their outputs, or an electronic switch (like this one) to select just the higher source (and maybe redirect the lower one to a battery). Even then you'll need to somehow regulate the output (since both batteries and loads will probably have an operating voltage range to be met). But those things would be a bit of reinventing the wheel, since you can purchase a ready-made charge controller to take care of these things.