Well the easier and probably the way that most people would do it, is to simply set your two routers to be access points. What that means is that they would act as switches, allowing your main gateway to serve the DHCP configuration to all the clients, so they would all be on the same subnet /network. Consult your routers documentation on how to set them up as AP's. Most routers can by be used as either. Just make sure that your main gateway allows enough IP addresses to cover all of your clients. And remember that AP's are connected / linked via a normal LAN port, do not use the WAN port. Hope this was useful.
The Cleaver
The Cleaver
