CS4 - Mask and Anti Mask

[coldpenguin]

I am not sure that what I am trying to do is possible, but I would like your thoughts.

I have an image, which has two people in it. I would like to turn the skin of one person blue, and the skin of the other green.

So I have added two Hue and saturation layers, mixed one to green, and the other to blue.
I then modify the masks, so that one colours the skin green, and the other the other persons skin blue.


However, I am getting parts, like where the hand is touching the other person, where the masks are mixing, and I am getting a purple.

So I am wondering, is there a way that I can set this up to be a single mask, covered by another mask?

I.e. I have a group, with a mask set tht exposes all of the skin to the contents of the group, and mask not set you see the background layer behind it.
Within the group, I have a mask which where set, the skin is green (or adjustment layer A), and then where the mask is not set the skin is blue (or rather adjustment layer B).

I know that I could in theory mess around with, apply mask to selection, but that seems a one-off solution, rather than something I can easily adjust later on.

 

[coldpenguin]

So, for a very simple example, an output is this:

which I am currently doing with two masks I am having to keep up to date:

If I want to move the boundary (if it is out of place), I have to remove from A, and then add the bit to B. Is there a way I can set it up so by removing from A, it automatically reveals B.

Whilst leaving the layers as adjustment layers (I could just duplicate pixels, but that will double the memory usage, and the image can be quite large)

What I want, is to create a mask like this, for the 'group'

then use a single mask like one of the ones above to change from green to blue.

 

[coldpenguin]

Ok, think I might have managed it.

Select image to use.
Create Hue saturation 1.
Create Hue saturation 2.
Create a group, entries Hue 1, and Hue 2, with Hue 1 above Hue 2.
Delete layer mask for Hue 2.
Right click on hue 1, Set knockout to shallow.

Now, edit mask for group, this should turn the skin to Hue 1.
Now edit mask on Hue 1, this should turn the skin to hue 2, NOT Hue 1 + Hue 2
woot

Any disadvantages to this, or better ways of doing it?