I am by no means an expert on DHCP failover; however, the ISC dhcpd.conf manual page states the following:
The failover protocol allows two DHCP servers (and no more than two) to share a common address pool. Each server will have about half of the available IP addresses in the pool at any given time for allocation. If one server fails, the other server will continue to renew leases out of the pool, and will allocate new addresses out of the roughly half of available addresses that it had when communications with the other server were lost.
Thus if one server fails, the second should still issue new leases from half of the address space. Whether or not this is working correctly in pfSense, I cannot say.
I gave up on using DHCP failover with pfSense as it will not work with dnsmasq/DNS Forwarder.