1. Yes, temperature control will help you get rid of phenolics and off-flavors. Depends on what you are brewing, but 74-78 is above ideal temperature for most ales.
2. You have a couple of options to control your temperature. If only cooling is a factor (i.e. room temp is always >70 F) then you can buy a one stage controller like a Ranco
. You can get it pre-wired so that all you have to do is plug it in and then plug your chest freezer into it. If I were you, I'd get a thermowell to get the internal temperature of one of your 20 gallon containers and put your thermocouple in that. With a 20 gallon batch, you'll create a lot of heat during fermentation, so you want to activate temperature based on temperature of middle of fermenting wort as opposed to internal temperature of freezer. The downside is that if fermentation between vessel #1 and vessel #2 is different, you're not accurately regulating temperature of both. Other option is to sandwich thermocouple between both vessels to give you a "average" temperature. I prefer to take temperatures with thermowells, though. I think they are more accurate.