Adding electrical outlet for wall mounted TV
If there are two cables in your existing outlet, one of them is supplying electricity to the box, and the other is connected in order to supply electricity down the line to some other outlets/devices on the circuit.  I expect the existing outlet to be pig-tailed with the conductors in the other two cables.

If you disconnect the two cables in the existing outlet (turning the circuit breaker off first of course) then I would expect only one of the two cables to be 'hot'.  The outlets before this will stay on, and the ones after it will be off.  If this is the case, then you know both cables are on the same circuit. You can choose either cable to be the supply for your new outlet.

Whichever cable you choose, you will:


Cut the cable
Run both sides of the cut cable into your new box
Attach the new box (probably use an 'old work' box)
Using a pig-tail, attach the black/white/ground together and to the new receptacle


You will probably run into the same problem I had, which is once you cut the cable you don't have enough slack to do the above.  If this is your case you will need to add an additional junction box above or below the new outlet, and cut the cable again.  There you will splice in some additional cable, then cover this junction box with a blank cover.  Since it will be behind your TV, it won't be a big deal.