Be very careful, to prevent data loss or corruption if both devices are
switched on at the same time:
- Using IMAP email accounts
- Using Synchronisation software
- Using Online storage, such as Dropbox, ownCloud, iCloud etc
- Doing anything too quickly with the above & changing/reversing (eg folder moves, copying & deleting)
- In particular, LogTen with Dropbox & 1Password with Dropbox
...and yes I’ve broken most of these 'rules'
IMAP
Results in getting locked out of the mail account, normally temporarily.
Sometimes the devices do not sync the accounts in a timely manner.
Don't think anyone uses POP3 any more, but this would result in mail on only one device.
Exchange Server (corporate mail), might get upset if you are appearing in 2 different locations
Backup before Sync, or risk an empty folder due to the 'newer' copy being empty!
Online storage is generally pretty reliable, but don't have the same file open on two devices (especially LogTen or similar).
If you drag and drop a (large) folder to the wrong place, let it sync online & upload fully before fixing this. Otherwise you could end up repeating the operation two or three times.
And the golden rule:
Replication/synchronisation is not Backup!
Good luck.