Second modem/router

Generally speaking, no.  You can't have multiple modems, modem/WiFi router combo units, or Comcast gateways at one residential address without jumping through hoops if it's even possible.  It would also be expensive as you'd end up with two accounts.

I'd suggest getting a mesh system if you're using your own equipment or xFinity Pods if you're renting from Comcast and don't have any trouble with 5GHz, 6GHz and 2.4GHz being combined on the same SSID.  Some 2.4GHz only equipment has issues with combined SSIDs and need the 2.4GHz to be separate.  That doesn't work with Pods.  If that's an issue you can get a WiFi extender.  I'd make a point to get one that supports a wired backhaul just in case you need it.  Same with the mesh kits.  Most will support wiring the nodes together, but some don't.  Earlier generation xFinity Pods didn't support a wired backhaul, but the current ones do.  If you go straight to wired you can also use pretty much any WiFi router (generally not combo cable modem + WiFi router units though) as an access point.  Business grade WiFi access points are another option, though the setup is usually a good bit more complicated.  All that said for most people the best options are mesh systems or Pods since the nodes will actually talk to each other and features like fast roaming will work.  Business grade APs can do all that and more, but require more technical skill just to get them working at all.

If a mesh system, Pods or an extender isn't enough to get coverage in the basement or you want more speed downstairs you could pull cat6 cable or buy a couple MoCA adapters (ethernet over coaxial) to let you run ethernet over that coaxial run and use a wired connection between a mesh node or Pod in the basement.  MoCA can do up to 2.5Gb.

If you go the MoCA route you'll need MoCA compliant splitters and probably a MoCA point of entry  (PoE) filter.  They're like $10 on Amazon.  The PoE filter needs to be installed where the cable comes in so it can block MoCA signals on Comcast's network.  Without it you might have neighbors connecting to your network via MoCA.  MoCA runs at a higher frequency than cable TV & internet, so the PoE filter just blocks higher frequencies and the splitters let them pass.  Splitters that aren't rated as MoCA compliant may let the signal pass anyway, so don't trust one as a PoE filter.  Comcast gateways often have MoCA capabilities, so you might be able to just get one MoCA adapter for the basement and let the gateway handle the rest.  If you're using a separate modem and router you will need two MoCA adapters.  Modems don't do MoCA.  Customer owned combo modem/WiFi router units might support MoCA.  Check the manual.

Hello, @user_a9beb5. As Zandor60657 mentioned, xFi Pods would be a great way to extend your Wi-Fi network if you rent an Xfinity Gateway. Here is a link with more information if you are interested https://www.xfinity.com/support/articles/xfi-pods-faqs. Generally, one modem can be active per account. 

Comcast is saying I can have two different modems at one address. That has two different billing names. But the corporate office said I can't So which

is it 

@user_0svocc 

Please create a new topic of your own here on this board detailing your issue. Thanks. The original poster has not returned. 4-month-old dead thread is now being closed.

For future reference, it is better to submit your own post as it creates a ticket to get help, and posting on someone else's older thread can delay getting help.