You can pair one key to as many Monimoto units as you like. For example, your car and your motorcycle - you can use same key since you would not drive both vehicles simultaneously. 

When pairing Monimoto with a new key that is already paired with another device - it's important to make sure that you only have that one Monimoto unit within Bluetooth range at that moment. Otherwise, you can not add the same key to the other Monimoto device.