Yep, very possible. You can use the SCP Upload action to achieve this. Here's a step by step guide:
1. First you need to turn on Remote Login on the Mac you want to send files to. To do this, open System Preferences and click on 'Sharing' then tick the checkbox next to 'Remote Login' - this will switch on SSH access to the Mac which also allows SCP file transfers:
2. In Dropzone 3 on the Mac you will be sending files from, you now need to install the SCP Upload action. You can install this by clicking 'Install' on the SCP action on this page https://aptonic.com/dropzone3/actions - note that this action requires the non-Mac App Store version of Dropzone. If you purchased Dropzone 3 from the Mac App Store then you can switch to the non-App Store version by downloading it from https://aptonic.com/dropzone3/latest and moving it to your Applications folder and replacing your existing version. It will detect you purchased it on the Mac App Store and register you automatically.
3. When you install the SCP Action it will show the dialog below. Click the 'Add to Grid button' (you can also add it from the add action menu in the top left of the grid.
4. You now have to put in the connection details as shown in the screenshot below. The Server field should be the name given underneath the 'Computer Name' field on the other machine from the first step. Username is your Mac user account username. Note that if you don't have a password set for your user account on the destination machine then you'll have to set one up and then put this in the Password field. Remote Path is the path to put files on the destination machine. If you just want them to go onto the Desktop for example then you can use the pattern /Users/<username>/Desktop. You can leave the Root URL blank:
5. Once you've put in all the details, click 'Test Connection' to check it's working and click the 'Add' button to add the action to your Dropzone 3 grid.
6. That's it, you're done - you can now drag files onto the Dropzone 3 menu item and then onto the SCP Upload action and they will appear in the chosen destination folder on the other Mac. If you want to share files from the destination Mac as well you can setup the same thing in the other direction.
Hope that helps! Let me know if you have any problems following this.