My second paragraph above references DNS servers, but I meant to say DHCP servers. DNS is a non-issue in your case since both servers are DCs, they will always run DNS and pretty much always be in sync (minus some offset for replication timing).
DHCP doesn't sync (at least not DHCP servers running on Windows), and has nothing to do really with DNS. DHCP works like this:
1. PC boots up, sends out a broadcast saying 'I need an IP address'.
2. A DHCP server hears the broadcast and says 'I can give you an address.' (called a DHCP Offer)
3. The PC says 'OK, give it to me.'
4. The DHCP server allocates an IP address and sends it to the PC.
5. The PC gets the IP address and says 'OK, thanks!'
Now, if the PC in question already has an IP address leased from that DHCP server, the server will not allocate a new IP address. It will just say 'Go ahead and use this IP address, the lease is still good on it.'
So what happens if you have two DHCP servers passing out IPs on the same subnet (let's call them DHCP1 and DHCP2)?
1. PC boots up, sends out a broadcast saying 'I need an IP address'.
2. DHCP1 and DHCP2 both hear the broadcast and issue DHCP Offers to the PC.
3. The PC picks one of the offers, usually the first one to come in, and accepts it. ('OK, give it to me.')
4. The DHCP server whose offer was accepted allocates an IP address and sends it to the PC.
5. The PC gets the IP address and says 'OK, thanks!'
6. The DHCP server whose offer was not accepted eventually expires the offer after a timeout.
So what if the PC still has a valid lease from DHCP1, but the DHCP Offer that comes in first is from DHCP2? Well, the PC will take the offer from DHCP2 and lease a new IP from DHCP2 instead of re-using the IP address that was leased to it by DHCP1. But since DHCP1 knows nothing about the offer from DHCP2, it doesn't expire the lease. Therefore this single PC can be leased two different IP addresses from two different servers on the same subnet.
There are undoubtedly third party DHCP servers that are designed to be clustered and service the same IP scopes using multiple servers for redundancy. But AFAIK Windows DHCP server doesn't do this.