Hi there, I'm Tim and will be assisting you today. I have over 15 Years as an Engineer and have been with eBay, DoD, and many others. I will do my VERY BEST to help you. If you like i can offer remote access and a call. In addition, with any positive rating I will support your question going forward for free.
the short answer is you can't. You can not have two active routers on the same network - period. You can turn DHCP off on one and put a VPN in on it, but i'm not sure that would be best. Using the single wwrt router wit VPN is a solid bet.