Signal has had backup on Android for a long time. On iOS you can transfer from an old iPhone to a new one now. You scan an QR code on your old iPhone with your new iPhone to exchange a secret for transferring your messages to your new phone. I don't think you can do Andriod<->iOS. You can have sync between desktop, phone, and iPad. However you can not have messaged synced to two phones or an Android tablet yet.