Two differents subnets in one DHCP Server with one NIC

If you want your DHCP server to receive client requests from other networks, you will need to set up a DHCP relay in each such network, and each relay will need to be configured to forward client requests to your DHCP server. I believe that the ISC DHCP package is capable of providing relay service, but I have never used it in that capacity. Many routers can also be configured to act as DHCP relays on networks they're attached to.

In your case, it seems logical to configure a DHCP relay on 100.100.68.1, since it's a router. However, any server (with a static IP) on the 100.100.68.0/24 network could just as easily fill that role.

(By the way, DHCP servers never broadcast, they always send direct (unicast) messages.)

We're doing this with Server 2003 where I am. The key is our central layer 3 device (in our case an old 3Com 4900 SX). It has what 3Com calls a "virtual interface" for each vlan. When dhcp requests arrive at the device from the outside it forwards them using same vlan interface they arrived on. The dhcp server just has a scope setup for each vlan with no other special configuration details. It matches the dhcp request to the originating interface on the layer 3 device to know which scope to use. That's out of the box behavior for the windows dhcp server.

The downside to this approach is that segment with your server must now be a member of all vlans (on the same virtual segment). This means all your broadcast traffic will now go up the fiber link to your main office (though not down all the other links to other branches), and it's nice to keep those links as clean as possible. It'll also use time on what is likely already your busiest local segment to reach your server. But this is probably unavoidable anyway.

Update:
Looking again, we do use the 003 (Router) option under Scope Options to match the address of the virtual interface with the correct vlan. That may have something to do with it as well, but I think that really just sets the default gateway.