Well, the way it works is that if I just put the two pictures next to each other, you'd have 2 different backgrounds, so to avoid this, I'd cut out the 2nd picture and want to put it on a canvas that has the first picture on it, but is wider, with the same background being on the extra width part where the second picture will be put.
Essentially, you take the first picture and need to make the background (what doesn't include her- the red whatever it is) and you need to "extend" it, meaning the result is the 1st picture, plus a lot of extra red background to the right or left. That's where I'd paste the cutout of the second picture. Hopefully that makes some sense.
Basically you can't just add the second to the first while making the whole thing wider at once (at least that I know of). I'll look into it and I know it's possible.