NO - from my experience, they do NOT stack coupons.....i.e. you cannot use the add'l 20% off with the tool box coupon.. However, if they run a sale in the store (or online) for less that their regular price, you can use the 20% off.
Since "Black Friday" is coming up............as they did about a month or two ago, they will likely have some kind of big 'sale" in which prices are dropped. Then, you can use the 20% off coupon.
As an example (and I missed this sale) they held a parking lot sale in early October. They had this small aluminum floor jack on sale, in the store....or parking lot, really

..... for $59.99. Minus the 20% off coupon it was $48. I recently bought one without the benefit of this sale, but used a coupon from a magazine for the sale price of $59.99. Couldn't get the extra 20% benefit and I knew I had missed the bargain sale price at the store. Oh, well.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=91039
You might wait a week and see what happens online, as well as, (and more importantly) in their stores. I expect bargains, per se. PLUS, free stuff! Always free stuff coupons..........lately, they have been a cheap screwdriver set, 25ft. tape measure and small aluminum LED flashlight. All with coupons of one kind or another, no purchases necessary. They have been found in Car & Driver, Motor trend, Autoweek, Popular Mechanics, USA Today (believe it or not), etc..
The deals offered in the ads mentioned above are typically BETTER than the fliers you receive in the mail or from their in-store ads. Don't know exactly why or the strategy, but they are.......and, even within this type of ad from one or more of the magazines, the same product might have three different prices in 3 different months or 3 different magazines. So, you need to be vigilant and compare.
Hope that helps, from my experience, anyway.
See ya.
Unfortunately, that's marked as a "super coupon", so I am guessing that you can't use the 20% coupon on top of the $339.99 price. Can anyone confirm/deny?