Simply add a two- or four outlet box to each line. Technically, one should advise you to have an electrician do it. Don't know what your local code is but it should allow at least 3 or four duplex outlets per line. Also consider what you have connected to each line and balance the load as much as possible. If you determine that your total load is more than your present lines should support, then, of course, another line is in order. If someone is going to object to splicing from the boxes to add more boxes on the basis of "broken " lines, consider that one duplex outlet in each present box is probably spliced from the other unless someone has bothered to keep the line continuous within the box (not normal for electrician) and that the connections to the duplex is contact with screw terminal. Six feet of 00 super cable from the wall is also questionable expense when you have 50 feet of standard Romex to the box. Oops, that's another topic.
Power Distributon - "More Plugs"
Curious to hear opinions on this one. I plug my gear straight into the wall. I have two 20amp dedicated circuits each of which terminates in a 4-box. I'm installing a house-wide surge surpression system (for lightning protection). I simply need more plugs! What would you do? I don't want power strips with surge surpression or anything like that - I just want more plugs on each circuit. Opinions?