[Gimp-user] Transparent background on flattened images?

[recoveringartist <forums gimpusers com>]

Hi, I've been making use of this forum the past few months as I learn GIMP, but
this is my first time posting.

I'm working with GIMP 2.8.16.

My question:  Is there a straightforward way to get a transparent background on
a flattened image?

The top portion of the book cover I'm generating features two separate images, a
girl and a guy, brought together to give the illusion they're with each other in
the same setting.  I want to set a third image (a blueprint vector) behind them,
then reduce the opacity on the first two layers so the blueprint lines show
faintly through.

Problem is, when I reduce the opacity on the girl, the guy shows through her
(see attached photo "Book cover transparency attempt").

The same thing happens when I use Merge Down.

I've experimented with opening a new view and flattening the girl and guy layers
to make one image.  Hooray, they're equally solid now and I can adjust their
opacity simultaneously with no ghosting.   But now I've got a gray, opaque
background where I need it to be clear.  (See image "Flattened view").

If I must go through all the _fun_ of redoing the layer mask, I'll suck it up
and do it.  But if there's a simpler, more elegant way to get a clear background
on a flattened image, I'll be immensely grateful to find it out.

Or . . .  is there another a method for joining two images together so their
opacity can be adjusted without one image showing through the other?  That would
solve the problem, too.

Thanks!

Attachments:
* http://www.gimpusers.com/system/attachments/620/original/Fullscreen_capture_712017_41303_AM.bmp.jpg
* http://www.gimpusers.com/system/attachments/621/original/Fullscreen_capture_712017_43823_AM.bmp.jpg

-- 
recoveringartist (via www.gimpusers.com/forums)