I've never tried adding two separate yeasts (except for other bugs like brett), but the problem I've always heard is that usually one yeast just takes over the other one, so you only wind up tasting one yeast rather than a combination of the two. That being said, I didn't think the 1010 gave off a lot of additional flavors anyways, so I'd just go ahead and use just the 3638, especially if you've still got some of the 1010 beer around to compare it to.
If you were really intent on using both yeast, the best way might be to ferment half the batch with the 1010 and the other half separately with the 3638, then blend them together at bottling time. Not sure what it would do in the end, and if you were to do that you could even bottle half of each batch separately, then blend the other halves so you'd essentially have three different beers to try!