If all else fails, there are current regulated bright panels designed for 10-30VDC operation. That's the ideal solution, since LED's are really a current driven device. But then they come with a price.
superbrightleds.com has such 10-30V DC panels.
It's worth a try running two 12V in series. May or may not work out. Reason again is that LED's are current not voltage devices and if the panels are not exactly matched, one panel may have slightly higher/lower voltage drop across the LED's than the other, so one ends up with more than 12V and the other less than 12V. If they are close probably not a problem. If it starts to deviate much, then you risk the higher voltage panel going into thermal runaway.
What I would do is hook each 12V unit up to a battery, let it stabilize for temp, then measure the current draw. Try to match panels that draw as close as possible the same current draw. That way you will at least start with "matched pairs".
the better you can heat sink them, the better off you will be. forward voltage drop is a very strong negative function of temperature, and as temp goes up, forward voltage drop for a given current goes down and with resistor limited panels that means current starts going up, power goes up, Vf goes down more..... Mex, I think you'll get the picture!!!
