Only one will be your main router and provide DHCP service to assign IP addresses for the network. The second will be configured as an AP. The easiest way to do this is to attach the two together LAN to LAN port with an Ethernet cable after configuring the AP unit. If you cannot run an Ethernet cable between them, you can use a pair of powerline adapters but that is not optimal, especially for gaming or streaming video.
On the AP unit disable DHCP, give the AP an IP address in the main router network range (it can either be a static address outside the DHCP range allowed in the main router or you can use address reservation if your main router supports reserved dynamic addresses). Use the same address for the AP in both the AP and in the main router (you will also need to enter the AP MAC address in the main router).
The AP unit should have the same SSID (network name), use the same security type and password, but use a different radio channel (selecting from the three non-overlapping channels 1, 6, 11).
All the settings on the AP should be done from a computer attached directly to it with an Ethernet cable, then when complete, attach the AP to the line from the main router.
The three unused LAN ports on the AP can be used to make cable attachments as they will act as a switch.