That's a limitation of any device supporting Exchange Active Sync.
Exchange administrators can enforce policies on mobile devices. It is possible to set up multiple Exchange accounts on the device as long as no more than one of them is attempting to enforce policies. If more than one Exchange account attempts to set policies on the device, it will result in a conflict and you will not be able to set up the second account.
The same goes for devices configured using other Mobile Device Management solutions.
A device can only be managed by one system at a time.