Hello,
you will run into trouble with 2 DHCP servers on the same physical network for 2 different domains.
A DHCP client will broadcast for an ip address and the DHCP Server answering first will mostly be the one the client will choose to get an ip address. But by default you can NOT state anywhere that the DHCP server only offers a lease to domain A and NOT
for domain B.
In your case one DHCP server must be removed and on one domain you have to work complete with fixed ip settings to prevent clients getting the wrong DNS server to reach the correct DC for authentication.
I assume you will merge the forests asap to get rid of one of the domains? Then ADMT is the, free from Microsoft, way to do this.