Two networks on one coax cable in the same residence?

My sister recently got a separate service at the same residence. I already have my own service. Thwt are both through xfinity. A coax splitter was used to splice between the two modems supplied from one coax connection in my living room. How does this work, for two networks in the same residence, without interference? I was under the assumption that you needed to have separate coax runs for separate services. Any information would be helpful.