Connecting a Side Car Modem to a Nighthawk CM1100 or CM1200

I'm looking to replace my rented Xfi modem with a Nighthawk CM1100 to connect to my Amplifi Alien router.  The issue is that I have Xfinity Home Security that needs the Zigbee hub integrated into the xFi modem.  Comcast will provide me with a side car modem at no extra charge, but I'd have to connect it to the Nighthawk modem.  The CM1100 does have 2 ethernet ports, but I've read that they can't be used to connect separate devices.  I'm looking for input on how to deal with that issue.