[stomachmonkey]

I'm going to assume the TW supplied modem is wireless.

They pretty much all are.

The Apple TV has wireless. Is it too far to get a signal from TW modem?

I would just drop a wireless access point or repeater somewhere in between to extend the range of the TW modem.

Cable modems perform best when they are the 1st device off the split. Adding another one on existing, unknown integrity, coax drop could result in degraded performance.

A second modem will also need to authenticate back to TW so while it's certainly within the realm of possible you may get hit with a charge. Don't know TW's policy on that.