Are you using the same account on the phone and tablet? If so:
You can sync your device with any compatible phone, tablet, or computer that meets syncing requirements. If you receive notifications from your phone on your Fitbit device, you'll need to disable the Bluetooth bond between your phone and Fitbit device before using a different phone.
In this case, your tablet. I've forgotten more than once to turn off Bluetooth on either my other phone or iPad, and my device refused to sync with my iPhone. Someone else can probably answer this better than me, but I don't think you can sync both simultaneously.
https://help.fitbit.com/articles/en_US/Help_article/1877.htm
