DD-WRT is very flexible so there are several ways of doing this. Since you want the maximum Wifi speed, you'll need to use physical CAT5 cables to "daisy chain" your routers:
Do a hard reset on the second router.
If you want to connect two routers with an ethernet cable, so that all devices connected to either of them can communicate with each other, plug an ethernet cable into the LAN (Not WAN) port of each router, set the IP to the second router to the same LAN IP address as the first router PLUS ONE (eg. 192.168.1.2), disable dhcp on the second router, and set it to a different channel as the first.
If you want to connect two routers with an ethernet cable so that the clients on one router are isolated from those on the other you need to use IP table rules to do this fully. However, you can do rudimentary isolation by plugging the ethernet cable from the first router's LAN port to the second router's WAN port, set the IP of the second router to a DIFFERENT Subnet, (eg 192.168.2.1 - Plus one to third octet if using 255.255.255.0 as subnet mask) and leave dhcp enabled on the second router.