Some odd ways to do it from your TV. For example I have a cable splitter set up to send the signal also to my TV which supports PIP. I then can watch one local channel via the TV's tuner, and one via the STB.
Like that other poster said it is possible but no like it used to be. When the cable signal was analog you could place a 2 port cable splitter behind your TV. The splitter would connect to both the A and B turner which would allow PiP.
Now with HDTV and digital cable you would still use the 2 port splitter and it would connect to first cable box>HDMI to TV and other feed would go to the coax input of the TV. While on HDMI input you would be able to see the TV input in the PiP. But it would be limited to local channels.
If you wanted PiP with all channels you would need 2 cable boxes one connected to HDMI and other connected to TV input. The issue with FiOS that all the cable boxes use the same remote so you would need an alternate remote control option for the second box.