The main DHCP-service will stop responding to requests while exporting the DHCP-database. That is maybe not a big problem, since it usually takes less than a minute.
The other problem is that while the socondary DHCP-service is importing the database it enters running state, leving you with two DHCP-services running at the same time while importing, that could take 10 minutes if you have a huge amount of reservations and leases.
I think that this will be a bigger problem than you wrote about.
It depends on how many new leases you have each day, if you have an enviroment with wery few new leases (not renewal of existing leases) you probably can handle the problem with these computers.
Consider how often this problem will occur, how often has your main DHCP-service been down?
But if you users want this option I can implement it in a later version.