Want to use RAX120 and X10 [R9000] connected to the same modem

Can you connect more than 1 router to your CM1200? 

Depends on your isp. Most isps (comcast, cox, spectrum) only allow 1 public ip address. That allows only 1 device to connect to the modem. Exceptions to this are when you pay for extra IP address or have a business class line. Which most people don't because it usually isn't worth the primce. 

But you don't have to connect them seperate. You can always connect the 2nd router to the primary and run the 2nd one in access point mode. 

The issue that you'll have is they won't be seamless. Neither device is a mesh router. So if you set them with the same ssid, they'll tend to be sticky. By sticky, I mean that devices will tend to stay connected to one of the routers until the signal is unusable or disconnects. Then it'll go to the next signal. its not seamless. 

If you're wanting true seamless roaming, you should look at the orbi line of mesh devices. They off wired and wireless backhaul with single ssid function. In a house your size, that'd be a better option that trying to go a little cheaper and cobble it together. 

I'd probably return the RAX120 and go with an orbi setup. You can always use the R9000 as a backup or even as a router or just IoT devices. (in access point mode)  If you do this, just make sure it uses a different wireless channel from the primary to prevent interference.