I usually assume the EGC in a cable is sized to 250.122. That may not be "full sized" and is significantly less in anything but the "small conductors" using 250.122 table. I haven't really analyzed the numbers to see how that would work when you double the ungrounded conductor size vs the EGC sizes in all cases.
Just looking at the easy example, 3/0 is good for 200a (@ 75c) and can have a 6ga ECG. Paralleling 2 of them gets you 400a and the required EGC is a #3. Table 8 says a #6 is 0.027 sq/in and a #3 is 0.053. Two #6s are 0.054 so it is pretty much the same with a little to spare. I still have not seen an issue.
