Adding a second power outlet to existing one.

Yes you can extent/add to the existing two duplex receptacles.If you don't use the original two,then you still get the benefits of dedicated lines,although I am sure the purists will say that some noise will enter the lines from those two unused duplex receptacles!.Or you could "kill the existing lines by terminating with marets" but leave the wires in the wall from those original two lines and run new wires to the new location,using the same breakers in your panel,since you don't have room for more.Another option would be to remove the existing two breakers and replace with two double breakers,so you will end up with 4 functioning lines.

Thanks for the reply Yioryos. Excuse my ignorance but what are "marets"? Also would the double breaker that you suggest fitting into the same space of my existing single breaker be able to be 20 amps each? and would they affect the polarity of current circuits which at present are numbers 1 and 3 which I think I'm right keeps the polarity the same for both .

Yes, the double breaker switchs would fit into the same space. I don't think the polarity would be effected.

There is no "polarity" with single phase AC(125-130V/60hz house wiring). You've got a hot(usually black) wire, neutral(usually white), and hopefully an earth ground. An outlet can be "out of phase" with regard to other outlets in the house, if it's not wired according to code(hot wire from the breaker on the outlet's brass colored screw, neutral on the nickle). You can have your electrician wire the new outlets to the same breakers, and if you're really worried about RFI/EMI being picked up by the old wiring: just have him disconnect it from the breakers. Adding ganged breakers wouldn't affect anything negatively, but you'd still have the possiblility of noise being picked up by the old wiring. Of course: everything that's on-line between the generating station and your home is contributing noise to your AC anyway. Get a Richard Grey Power Company, Chang Lightspeed, or Audio Magic Stealth XXX(what I use)power conditioner, and you won't have to worry about noisy AC anymore.

OOPS: Make that- neutral on the "nickel". Some further info: (ftp://rtfm.mit.edu/pub/usenet/news.answers/electrical-wiring/part1)