If your isp even allows two connections to one pc.. you can do it.
If your isp allows more then one router to be connected to that subscribed line.. it can be done. But usually one router is used, and the other turns into a switch.
If your isp and /or router (multiple-WAN/gateways & load balanceing) does not support this.. load balanceing or not.. you'll connect to one or the other..OR both will be limited by your isp to a set speed of their liking..or you'll get kicked and powercycled until one router is on the line.. Please check with your isp if they allow you to do this first.. you can void the contract in some areas with certain companies..like comcast..but it can be diffrent from state to state etc..not all franchises are the same..
In that case your only option is to subscribe to two isp's, or upgrade to a small -medium business subsciption. Or ask your neighbors to use their wifi..