Photoshopping Two Istock Images together

Sure you can do that and this is one of the uses that stock has. There are only a few restrictions as laid out in the terms. As @PaintedKitty said, the underlying work still belongs to the original artists, so you will still be bound to the licensing terms. However, the modifications are your (c) and you can do that for a paid customer work.

Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788
(Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).

You can do what ever you want and use it for personal reasons. Just don't try to resell it for stock. That wouldn't be fair to the original artists.