This is a whole new can of worms. You are working off a PE's drawing, you should bounce it off of him/her first. If you are parallelling a pre-sized wire, you must meet the ampacitiy of the original wire. a 600 kcmils is rated at 420 amps at 75 degree rating. 1/2 of that is 210 amps, so a 4/0 would be the minimum which is rated at 230 amps. With the derating of six conductors, you are at 368 amps or 88.3 kva. To meet 100% capacity of the 600 kcmils, 420 x 1.25 / 2 = 263 amps or 300 kcmils. Now you may be looking at a larger conduit(s) In other words, I suggest you contact the PE first. He stamped the drawings so he/she assumes the liability of the design. The PE may have taken in some consideration for some future expansion or some heavy load that you may not be aware of.
Another issue you can run into is whom is paying you to do the work. You are attempting value engineering and the one holding the money may want a piece of the savings.
I may be making mole hills out of ant hills however, I do not know all the details. These kind of things need to be considered prior to deviating from a stamped drawing.
"Live Awesome!" - Kevin Carosa