Yes, you can use as many gateways as you like for a single account. They can be in the same location to expand the wireless coverage area, or they can be in entirely separate locations.
To add an additional gateway gateway to an existing account, simply add it to the app just like the initial gateway. Learn more about gateway setup.
Everything else is designed to just work seamlessly. You don't need to specifically assign sensors to individual gateways, etc. The gateways will report any sensors they see that belong to your account, and if there is overlap the system will keep itself sorted out automatically.
If you do want to know what gateway(s) are seeing the various sensors to help inform your placement choices, look at the summary section for a particular sensor (located beneath the sensor data graphs.)
If you have multiple gateways and a sensor has recently been reported by two or more of them, you'll see multiple lines, listing each gateway with its individual last signal strengths and times.