OK, now the problem here is which device you synced first. To not lose your data you had to sync the main device first the one that has all information and then the second device so it can pull the information already in the cloud from the main device. If you did this the other way then the main device pulls the information from the other device you synced first which obviously since it has no information it pulls nothing but it replaces the information you had before with nothing. (This is why they asked you to backup data first in case you make any mistakes.)
The only thing you can do now is check the Dropbox account to which you synced the devices and see if the folder is there and if it has information in it. If it does I will suggest you download that to your computer to have a backup of it and then try syncing the main device again.
If the folder is there but is empty unfortunately that would mean the information is lost as there is no other way to recover it.