Aha! tutu yarn. LOL. Actually, plying 2 two-plies together is a recognized technique, called "cabled" yarn. You ply the two 2-plies in the reverse direction than the 2-plies have, so you'd go clockwise again for standard yarn (see the post Why Z and S for why this is clockwise...)
That said, if your 2-plies were balanced plies and you are re-plying to make something new, you'd want to add some twist to the plied yarns first, so they have enough twist to be balanced in the cabled yarn.
And that said, something to consider is the amount of twist in the original singles -- if you are adding more twist to the plies, that may be over-untwisting the underlying singles. Usually for a cabled yarn, the singles are given a little bit of extra twist so they end up straight in the resulting cabled yarn. So, sample a little and see how it looks.
I've done this myself -- with some cotton -- the 2-ply was too thin and underspun in spots, so I added alot of twist to it and then cabled it. There wasn't any way to add more twist to the singles, and it worked out just great -- yay! so perhaps they really don't need more -- does anyone have a rule-of-thumb for the singles of cabled yarns? more twist than for singles in a 2-ply, less, or the same?
Given I'd already set the twist in the 2-ply, after I added the extra twist, I used the warm-bath method on a folded-back 6 inch sample to see how much twist the cabled yarn would need -- put a 6" folded back sample in a warm sink and it will twist up on itself to show you the balanced ply-twist needed; this works great for old singles too. I used that sample as my eyeball-test as I cable-plied the rest of the yarn. And, it looks great as a cabled yarn!
My other cabled yarn was a Romney; this was used as the cuff in a felted clog ... Note that cabled yarns tend to be fairly high-twist, and so don't felt as much as a 2 ply of the same thickness -- the edge of my clogs didn't felt as much, but it all worked out okay -- it turned into a sort of ruffle. Where are they now? My Dad took them home with him last time he visited ... sometimes it's not so great that we have the same shoe size!
(based on a post by me on spindlers, 3/2/2007)