Creative

The modem signal is mixed in amongst the frequencies used for the analogue and digital signals. So the signal will be available on both wall sockets.
The trouble with splitting a signal is the two resultant signals are now at half the level of the original. The STB has a window of approx 10dBmV, not sure about the modem but its likely roughly the same. So it would depend upon the level available at the wall socket output, as to whether splitting it (and hence losing 3dB) the level is still sufficent.
Additionally connecting your TV directly to the cable feed may mean you are sending noise back out into the local network. The frequency of the noise will depend on the analogue TV signal your TV is tuned to, and changes as you change channels.