Connecting two Comcast networks together in the same building

Just looking for some heads up on this idea I have... I have two office locations for the same company in the same building. Each has a seprate Comcast Business account / service.

I want to connect the two "Networks" together by running an Ethernet cable from one office to the other through the cealings so that the two seperate networks can communicate. 

In this case, each network is a simple Pier to pier network, so there will be no real servers running. At best, I am using Microsoft Azure, but this will not control network connectivity, only common authentication.

My assumption here is that once I have the actual Ethernet cable running from one office to the other, I would need to:

1. make sure the IP scheme and DHCP is different for each office.
2. Add a second Router on each end of the connection with NO DHCP
3. configure each new router as a secondary gateway connecting each office, filtering out DHCP, Internet traffic, Etc.
4. Preserve each existing Comcast router to be the default gateway for all other services for each individual office like DHCP, Internet, DNS, Etc.

I know it sounds simple and I have been doing I.T. for quite some time, but this is a unique situation I have not run into until now. I just want to make sure I have not left anything out, AND if this is going to cause issues with Comcast as far as any restrictions they might have for their networks.

Also, If I change the Subnet mask for IP4 on one router, do I also need to reset ip6 on that same router

Thanks in advance