Hi Troubleshooter24 and everyone,
Assuming, as you said, you have two branches connected by a VPN, let's call them B1 and B2.
Let's say that B1 is which will be giving DHCP leases for both B1 and B2.
Configure a new scope for B2 sub-network (e.g. 192.168.20.X)
In the B2 office you will need to stop any local DHCP server.
Configure a feature called <b>DHCP Relay Agent</b> so it passes any DHCP Request to B1 office. Considering you already stopped the existent DHCP server (in B2), the DHCP Relay Agent is usually done in the local router (in a cisco shop it'll be dhcp helper address). Once the DHCP Relay Agent is in place, pointing to the DHCP server in B1 you will start to get DHCP leases from the B1 office.
For fail over you have various possibilities. The first I can remember although might be a little overkill is clustering. For your scenario, the overhead, licensing costs, etc. it might just not justify.
The one I usually adopt in small to medium installations is a 50/50 DHCP scope installation.
In a 50/50 scenario 50% of each scope is provided by each DHCP server:
You will have:
B1 Office Scope = 192.168.10.1 to 192.168.10.254
B2 Office Scope = 192.168.20.1 to 192.168.20.254
DHCP Server 1
B1 Scope: 192.168.10.1 to 192.168.10.128
B2 Scope: 192.168.20.1 to 192.168.20.128
DHCP Server 2
B1 Scope: 192.168.10.129 to 192.168.10.254
B2 Scope: 192.168.20.129 to 192.168.20.254
This way each scope in each server does no overlap with the other.
Fallow up if you need some more clarification