Pairing multiple iOS devices with one bluetooth keyboard
Whilst you can pair one master with many slaves, as far as I'm aware, you cannot connect one slave to many masters simultaneously.
You need to un-pair &amp; re-pair each time.

There is a rather complex guide to piconet &amp; scatternet technology at the Bluetooth Developer Portal

Bluetooth v4.0 spec distinctly says it cannot be done, however v4.1 says it can - so it may depend on which version is used in the device.
http://9to5mac.com/2013/12/05/bluetooth-4-1-will-bring-more-reliable-automatic-connections-multiple-roles-more-via-software-update/