Connecting my mac to two external monitors. How can I stop the dual monitor from mirroring each other?

Hi, I am trying to connect my mac air to two external monitors (one is Dell U2419HC and the other is Dell U2419H). I connect my mac to U2419HC since it got the USB-C port and do not need extra display port connection. And i connected the two external screen using display port. The primary external screen is working perfectly with the Mac, i.e., not mirroring each other and functioning as the extended screen. But the two external monitors keep mirroring each other, despite the fact that the Multi-Stream Transport is on for both monitors. How can I stop the dual monitor from mirroring each other? Thanks!