You will not be able to connect the 2 DIR-878s wirelessly to each other. If you can use an Ethernet cable, then you can connect to the 2nd router to its LAN port, not the Internet port. You will need to disable UPnP and DHCP on the 2nd router and assign a static IP address since it cannot receive DHCP from the first router.
It is best to return one of the routers and buy either a range extender or an access point.