Networks

First, set up your separate domains. Each domain will need its own DHCP and DNS servers, DC's, etc., for its subnet.

If you want the users in both domains to be able to use resources in both domains, you need to either configure them as subdomains of the same forest, or create a permanent trust relationship between the two domains. How you do this depends on how interdependent you want these domains ultimately to be. If you create two separate forests/domains with a trust relationship, it's very easy to break them apart. If you create one forest with two subdomains, it can be complicated to then split them apart if you need to.

Set up two routers, one for each subnet, to route between the two subnets. If you're using two separate Internet connections, you would set them up separately on the two routers. This makes the routing a bit more complicated, as you have to set up a static route from subnet A to subnet B. If you are using only one Internet connection, you can simply route from one of the routers through the other one to the Internet.

I would recommend having a DHCP server in each subnet rather than trying to configure one DHCP server to handle everything. DHCP packets will not be forwarded by the router unless you specifically configure it that way (if the router has that capability) or set up one of your servers in the non-DHCP subnet to be a DHCP proxy.

As far as the actual IP address range for each subnet, I would be more inclined personally to stay as simple as possible:

Subnet A: 192.168.1.0 - 192.168.1.255, default mask of 255.255.255.0

Subnet B: 192.168.2.0 - 192.168.2.255, default mask of 255.255.255.0

When you add another subnet, just keep going with 192.168.3.0 - 192.168.3.255; etc.

(cont'd in comment)