Depending on your router and clients, you might find that the loads won’t be balanced. Since a normal Pi-Hole setup can easily handle a home network, this shouldn’t be a problem, but you may not get the balance feature with two Pi-Holes in parallel.
Example - I use an Apple router for DHCP, two Pi-Holes in parallel. Pi-1 is listed first in DNS, Pi-2 is listed second. Pi-1 routinely gets 99% of the traffic, and only a few IOT devices leak over to Pi-2. When I fiddle with Pi-1 for updates or the like, traffic moves seamlessly to Pi-1 and then back again.
For syncing white and black lists (I don’t use the audit feature), I run teleporter every few weeks. Since Pi-1 gets almost all the traffic, it has bigger white and black lists, so I export from Pi-1 and import at Pi-2.