Yes, you can absolutely do that. There is already a splitter someplace along the way that split the incoming signal to feed the modem and the other TV(s) in the house. You should be able to get a decent splitter at Wal-Mart, Lowe's, Home Depot, etc.
Because the Cable TV system is a "broadcast" setup, it doesn't matter how many devices are on or in use at the same time in your house. You can use the modem and any/all TVs at the same time without issues. The cable modem uses a different frequency on the cable than the TVs anyways, not that it matters, just more info.
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