There IS a way to duplicate save states on the 360 with an exploit in game, I can't really express it so well in text but hopefully this works
1 - Required. If you have another friend that is hosting a game publicly where you can join them from the dashboard. Join them. Can be a invite into a private match too you just need to be able to join someone.
2- As you go through the cut scenes and whatnot, when the dungeon defenders press start screen is there for a few seconds make sure to press start asap. There is a catch towards it, you can stack the prompt to select a device TWICE when you do this when you load into their tavern. So you have a few seconds to setup the first one, and the game auto boots you into their lobby, Which in return brings up the 2nd select device prompt. Make sure to not back out of this device selection menu untill next step. This 2nd one pops up since the game automatically brings it up when you didn't load one when you first booted up the game.
3 - When you are in your friends lobby. Select the save you want to duplicate first, THIS loads it up but after you do load it up the 2nd stacked save state prompt pops up which in return you can use it to select another storage destination (make sure that the 2nd drive is empty)
4 - Do something in-game that makes the game save, What I like to do is equip an item at the very least, it has to deal with something that edits the item box so fyi. This saves your loaded save onto the 2nd device you selected
5 - Exit the game safely and done.
You'll have duplicated the save. I found this out pretty late overall which is a more lazy way to backup my save in case I don't want to use horizon to store on my computer
Notes for a corrupted save state - Make sure your save state size is 150 KB on the dashboard, and that is only required with the first one, The backup for the most part can be ignored because that is just to keep characters etc in case the first save is gone / corrupted. IF it is 100 KB for the main save on your dashboard it means its corrupted / damaged so you have to manage to lookout for that when you think you potentially corrupted (IF you have more things on the save more likely it is going to corrupt so try to only keep things worth saving) I used to corrupt a lot since I had full item box / floor and 32 characters glitched in with another exploit in game lmao so saying from experience
Horizon is a really sketchy tool which DOES have some sorta adware virus from the last time I tried it out so like its definitely not great to have... (official site download ofc)
Also No. You can't copy a save state through the vanilla dashboard from the last time I played