Router – connect 2 switches to a router, what’s the difference between creating 2 subnets and creating 2 “normal networks” with their own net ip

The only thing is that you're not saying "all addresses should have access" -- that's done in your firewall(s) and/or the server software and/or other security layers like tcpwrappers.

0.0.0.0, in this context, means "all IP addresses on the local machine" (in fact probably, "all IPv4 addresses on the local machine"). So, if your webserver machine has two IP addresses, 192.168.1.1 and 10.1.2.1, and you allow a webserver daemon like apache to listen on 0.0.0.0, it will be reachable at both of those IP addresses. But only to what can contact those IP addresses and the web port(s).

Note that, in a different context (routing) 0.0.0.0 usually means the default route (the route to "the rest of" the internet, aside from routes in your local network etc.).

If you are only concerned about DHCP availability, then don't bother with a separate subnet. There shouldn't be any problem at all setting up 2 DHCP servers on the same subnet as long as they are handing out different ranges of IP addresses. It looks like your existing hardware should support it and it would solve the primary problem you mentioned. It also has the added benefit of some redundancy in case one of the routers stops handing out addresses, which seems common with little appliances like these. If you continue to run one subnet, remember that your machines will pick up addresses from either DHCP server.

There are other benefits to separate subnets such as limiting your broadcast traffic and the ability to use a routing protocol to provide redundant links in the future. Before adding this complexity, I would figure out whether limiting broadcast traffic will provide a noticeable performance improvement, either now or as your network grows. If you are going to grow significantly in the future, it might be better to just break your subnet up now rather than having to readdress a huge chunk of your network down the road.

If you do go with separate subnets, you will have a different gateway for the machines on the second subnet and obviously all of the routing to get between subnets. I also expect you would need to set up and use the WINS server that comes with SAMBA. (SMB doesn't play nice on separate subnets without WINS)

And as a quick aside, I have Cisco and RAD bridges linking several fire stations up over distances of several miles, and I can't say enough about good antennas. You might find your bridges to be a lot more reliable if you can get some dish or yagi antennas.