Parelling solar panels.

I don't think this fits the 'alternative' or the existing sticky on solar panels. I currently use a 40watt panel which I simply pass through the heiki rooflight and sit flat on the caravan roof once on site prefering the security to peak performance. I simply run the connection outside the van to a connector inside the battery compartment then via a controller to the battery and it is suitably fused. I have obtained a second 20 watt panel but am unsure of the physics regarding whether or not I can simply make a parallel connection at the charge controller terminals...and whether this might inhibit the original due to non-matching voltages...or worse. Any scientists or techies out there please who can advise or suggest alternative approaches please.