Two WIFI network with one Router

What are you trying to accomplish here?

While a guest network provides a second wifi access point, it's not clear to me if your intent is to establish a separate network, one that keeps users of each separated from the other, vs., just adding another wireless AP, but using the same network.

IOW, without understanding the intent, it’s not possible to say whether a guest network will or will not meet your needs. A guest network is typically used to create a second (public) network, one that allows internet access, but prevents those users from accessing your primary (private) network. But if your intent is to establish two wifi networks to the SAME network (for example, if you wanted to provide both a 2.4GHz and 5GHz AP), a guest network is not the answer. You need a second WAP (wireless AP), whether that’s a standalone device or wireless router reconfigured as a WAP.

So again, what are you trying to accomplish here?