Copy from Threema-Homepage:
coI've got a new phone — how do I move my ID/messages/contacts?
ID: You can create a backup of your ID within the app, and we highly recommend that you do so even if you don't plan on buying a new phone, as otherwise your ID will be lost forever if you lose your phone or if it gets damaged.
Simply go to the “My ID” tab in the app, scroll down, select “Backup” and choose a password. The private ID data (i.e. your private key) will be encrypted and displayed in text format and as a QR code. You can then copy & paste the backup text to a safe location, or scan the QR code using the new phone.
Note that if you perform the backup multiple times, the output will look different each time; that is because some random data is used in the password hashing process.
To restore, tap the “Restore” button on the screen where it prompts you to move your finger to generate random data.
Messages/Contacts: the procedure to follow depends on whether you're using the iOS or Android version.
iOS: messages and contacts are included in the iTunes/iCloud backup. If you restore your new phone from a backup of the old phone, you will keep all messages and contacts. Important: the ID needs to be backed up separately (see above).
Android: messages and contacts can be saved using the “Backup Data” option at the bottom of the “My ID” tab. A password protected ZIP file will be created in the path Threema/Backups; you can copy this file to the new device (in the same path). The restore process can be started using the “Restore” button on the initial setup screen (where it prompts you to move your finger to generate random data).
Messages and contacts cannot be moved across platforms (iOS/Android).
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Can I install Threema on multiple devices?
At this time only one device per ID is supported. While it is possible to transfer the ID to multiple devices using the backup/restore function, only one device can be logged in at any given time, and new messages are only delivered to the last device that has logged in.
A multi-device solution (with synchronization of sent and received messages between multiple devices) is currently in the planning stage for a future version.
I can see this thread is very old, however the recommended method of ONE person using TWO devices w Threema is to .....
1. Obv each device needs its own license & SEPARATE / distinct ID.
2. Both devices of this one person needs to be added into the same chat group.
3. Recommended to give each device/ID its own Nickname to distinguish the accounts. Ie Nickname according to "device as well as person".
Obv each device needs its own license & SEPARATE / distinct ID.
Hello 
yes, as of today on each device you need a separate Threema ID, but no separate licences.
Both devices of this one person needs to be added into the same chat group.
Correct.
Recommended to give each device/ID its own Nickname to distinguish the accounts. Ie Nickname according to "device as well as person".
Correct.
Regards Ingo
yes, as of today on each device you need a separate Threema ID, but no separate licences.
Depends. This is true if you use it with same Google or Apple account or Family Share.
But not if you use a Threema Shop Version or mix Google and Apple Devices.
