Licenses are applied to Orgs not devices.
If you purchased another MX64 to use in the event of a failure you could simply have the device sitting in the inventory (not assigned to a Network). If the time comes you can simply remove the faulty MX from the network and add the new one from inventory to the network in question.
On a side note, if you going to the expense of purchasing another MX for business continuity why not create a HA pair (2 x MX64 - Active-Passive). One of the good things if you go down this path is that you don't need another license. Only 1 ENT or ADV license is required for MX HA setup. You'll need to be able to span some vlans/ports on a switch to cater for the connections but at least you will have automatic redundancy.
Hope this helps