Can I make two Wi-Fi networks talk to each other?

In my house I have 3 wireless routers that all talk to each other, some are wired together and connected to a hub in the basement others connect via wireless. The main router is in one office on one side of the house that for now handles DHCP and broadcasts both the 2.4 on channel 1 and 5GHz networks and along wiht their corresponding Guest Networks. Then there is a router in the basement behind a TV that rebroadcasts the 2.4GHz N network for the rest of the house on channel 6. Then there is another router in my office on the second floor that connects via wireless to the 5GHz network that surprisingly reaches to the office the only device that can see the 5GHz network anymore than 2 rooms away is the router in my office, and that router connects a printer and another computer and also rebroadcasts the 2.4GHz network again and this time on channel 11 and this works because the router is dual band. Every router on my network has DD-WRT on it except the wired switches but they aren't supported... This network setup works rather well, they only difference between your setup and my setup is that I only have one DOCSIS 3.0 modem from Comcast supplying the internet. I have one more router that I want to add to my garage because I still cannot get wi-fi in there I can get it right outside and right before it. However I am out of non interfering channels so I am just not going to bother for now...

My house is around 5,000 sq ft so I don't know how that compares to yours but my setup works well and get decent signal through most of the house.

Why would you want to have 2 separate Internet connections? If you don't need it and can get just one big one you will have a much easier time setting up your network. Otherwise, im not sure what would happen if you had two connections.

you should be able to bridge the two routers with a crossover cable so that they can talk to each other... you would then set the DHCP servers on each router to supply IP addresses from different pools ( like 192.168.1.50 - 100 and 192.168.1.150 - 200 )(both with the same subnet mask) also you must set each router to a different IP address (192.168.1.1 192.168.1.2) When you connect to either router they will set your gateway to be that router so you will not use the other's internet connection... you should then be able to see the shared network resources...

As the routers will be hard wired together, giving you only one network, you will not require any VPN software or any such thing. You could also, if you wish, manually set each devices IP address instead of using the wireless DHCP servers and then connect to any router and not have to worry what one you are connected to... That means if your dad's signal is stronger you could connect to his router and still use your gateway(isp) and not use his internet service. You could also just use a single wireless router and a wired router this way... by setting the IP configuration manually you and control what device uses what internet connection no matter where it is in your network

If you are going to have a single internet connection and 3 routers then you can have all shared. simply pick the router (of the 3) that has the best specs (mainly related to CPU and RAM), then set it as the main router. After that, run ethernet cable to each of the other 2 routers, then disable the DHCP server of those 2 routers, then also change their local ip to a higher number, eg the main router is 192.168.1.1, router 2 is 192.168.1.2 and so on.

after all of that connect the 2 secondary routers to the main one LAN to LAN (and not WAN to LAN) this avoids having multiple NAT's and allows single NAS to be placed anywhere on the network and every PC connected to any of the routers will have access.

this is the simplest and least overhead heavy setup for multiple routers in the home.

I currently use 3 routers and this method in my home (with the main router connecting to a switch. Since I mes around with firmware a lot and other people in the house still need internet, I have a saved config file for each router to make it the main router and when I want to mess around with firmware, I can load up the needed config file, then simply disconnect and reconnect 3 wires and in under a minute the internet is back up (since almost all netgear routers use the same adapters, it is very quick to do)

If you have 2 separate internet connections, then sharing a NAS will be more complex and with have a major performance hit for all secondary routers.

Sharing a NAS across 2 WAN connections required you to set up a VPN, if your router are compatible with the tomato firmware, then you can either set up a openVPN connection (more secure but very complex to set up), or a pptp VPN server (extremely simple (you just need to create a user name and password (and either use your IP address or set up a dyndns or no-ip account)

Sharing a NAS over a VPN will limit the overall NAS performance to the upload speed of your internet connection

So you mean to bridge them to make one super connection? That's possible. Set one of the routers as the base, then the rest as repeaters, and you should be good to go.