I work in a lab with over 800 modems that can be switched onto a single coax feed (lots of splitters and forward/return amps). There is no problem with this and it will work fine. An HFC (hybrid fiber coaxial) cable plant is nothing more than a large splitter/combiner arrangement with amplifiers placed throughout the system.
If your friend was talking about the network aspect of it, it's hard to load balance a single data connection across two modems (say a single FTP download) like per-packet load balancing. You can easily do per-connection load balancing though, where each data stream gets a different path. I've done this in the past for business customers that had multiple modems.
The bandwidth is another point. a single 4 channel modem can use 100% of the available bandwidth on a cable plant, (if load balancing isn't set up properly by the provider) so there is an argument that there is no difference between 1 modem, and 2 if the modems are uncapped as far as downstream uncapped traffic goes. Upstream uncapped traffic can be limited by the upstream scheduler on the CMTS (around 8-9Mbps on most Cisco and ARRIS CMTS if upstream channel bonding is not enabled),