Novation advise that users avoid plugging or unplugging the jack cable that links your two Dicers while they are connected to your computer's USB port. This can damage both units by breaking the linking port, so you will only be able to use one Dicer at a time.
Connecting and disconnecting the Dicers to the USB port of your computer while it is powered up is absolutely fine and will not cause any issues.
This issue is covered under warranty. Please contact Novation if you need any assistance.