It's not currently possible as we've designed the Show Desktop button to behave the same as the Windows one. I've added the request to our feature request list though.
Thanks for the feedback, it's still not possible, but we'll keep it on our list in case we're able to find a way to do it in the future.
I'm curious why not. You need to have a list of all the windows on each monitor to place them on the correct taskbar, so why can't you just send minimize messages to each window on the monitor X list?
Ok I should clarify a little. It's not possible with our current implementation due to some limitations in Windows, but it is on our list to do, it's just going to take a significant amount of work. I've added your vote to the feature request Daniel.
Would like to see a fix to this. "Show desktop" only works for my primary monitor.
Same problem here. W7x64, displayfusion 5.0.0 (Beta 6)
Well in fact hovering over the button shows the desktop on both screens, but clicking on it only minimizes the windows on the screen with the original taskbar.
@Oliver: I wasn't able to reproduce this issue. Could you list the applications that are open on each monitor, so that I can try to replicate your scenario as closely as possible?
Keith, it happens with any application. I've even tried a minimal setup with one Windows Explorer on each screen to rule out any application related influences.
Ok, strange! I definitely can't reproduce it here, even with just two Explorer windows open. Could you attach a copy of the info from the DisplayFusion Settings > Troubleshooting tab? Please attach it as a text file.
Thanks Oliver. I've compared your Troubleshooting info with mine, and with another customer that's having this problem, and still can't spot any differences that would be causing this.
If you press Win + D, do all of the windows minimize, or still only the ones on the primary monitor?
Since last check I uninstalled fences 2, installed fences 1 and rebooted my machine twice. So it might be related to any of these actions. Hope this helps with your other customer.
Thanks for the update Oliver, that appears to be it! I tested with Stardock Fences and DisplayFusion, and it seems to be the combination of the two that is causing this issue. We're going to investigate further, and I'll post an update as soon as we're able to fix it up.
Well, I found another problem with that button. I just installed v5.0.0, Beta 8 so it might not have existed before, but I'm not really sure.
I've pressed on the show desktop button to hide all windows. Then when I slide the mouse over the DF show desktop button (this does not happen on the standard button), my windows get visible for a short moment and then redisappear. The same happens when I move the mouse away from the button. It looks like DF likes to do the peek at desktop action, but does not recognize that the windows are already gone.
Just a quick update, we’ve looked into the issue of show desktop only working on the Primary monitor and it appears to be a conflict between DisplayFusion and Fences. Unfortunately there’s nothing we can do from our end, but we’ve contacted the Stardock Fences support team to let them know about the issue, so hopefully there’s something they can fix on their end in a future update.
We've found the cause of this issue, and it has to do with the order that programs get loaded. Here's a FAQ article that describes the scenario, and a couple of workarounds. The article mentions mouse hooks, but the same goes for other types of hooks (like the ones that Fences uses for hooking desktop stuff)
Hey Keith. I'm keen to purchase this software but I need the functionality for the single monitor/show desktop mentioned in this post, did you manage to get a resolution or is it still unavailable? thanks
DisplayFusion will do that if it thinks there's a touch device (touch monitor, or graphics tablet) connected to the machine. You can override this in the Advanced Settings though. I've attached a screenshot
screen: 
