Installing additional outlet to the same line

I know, it sounds rudimentary, but I don't want to make a mistake.
I have two dedicated power lines going to the same wall location, terminated with double outlet each. I need to add one more outlet to each of the two lines, so I'll have 4 total outlets connected to 2 lines.
I have replaced outlets before, so I do know how to do it, bu the wiring diagram, I'm not so sure about.
Thanks in advance