Two routers will not double your download rate or even upload rate. You can use the second router as a switch to add more ports (or as a wifi access point) for your current router (and network). Simply change the secondary router’s IP address subnet to reside outside of your network.
So if your current LAN IP range is: 192.168.1.100 to 192.168.1.250. Then your second router’s IP should be 192.168.X.1. Change X to be any number other than 1 (so as not to cause a conflict with your current router. Then simply disable/turn off the DCHP server on your second router (it’ll be using your first router’s DHCP as the primary. Reboot all your devices and you should be good to go.
If you want the second router to act as a AP (access point), simply follow the instructions above and leave the wifi on the second router on. I would change the SSID of the second router’s wifi so you can easily discern which router you are connecting to (helps with troubleshooting).