20v solar panel powering two 12v fans and a 6v pump (swamp cooler for burning man)
I wouldn't recommend connecting the panels in series in case the 20V panel pumps too much current through the ones that don't work very well. To decide how to connect them (if it's possible) details of those panels would be required.

However, for the 20V panel I would say you need a switch mode buck regulator that takes the 20V and reduces it efficiently to 12V to power the two 12V motors. At 0.8A total draw from the two 12V motors the power requirement is 9.6W. The panel and regulator should be able to supply 90% of 20V x 0.8A = 14.4W.

This leaves about 4W (conservatively) for the 6V motor which you imply requires 0.6W. You'll need a second step down buck regulator for this device and it sounds feasible.

Switch mode buck regulators - there are tons of them but I can only recommend a design type such as the LT3976: -



For a slightly more efficient regulator there is the LTC3780 which will work providing 12V down below 12V from the panel: -



Both regulators can supply 12V at 5A. 

I wouldn't bother thinking about a linear regulator because it will waste about 8W in heat.

It's possible that you can search along these lines and get some kind of module/PCB that you don't have to build. Maybe someone can recommend something?