I undertand
This is possible
As Phil mentioned, you should make mix to transparent.
Open mix-options
Select mix 2
For mix Type - slect 'flat locked to corners'
background colour = transparent
(adjust the mix size as required)
When you fade out layer 2 (in mix 2) it will go invisible and you will see whatever is in mix 1 (no black hole)
In the jpg attached it shows the setings for mix 2. I have set layer 2 at 60% to show the transparency. This was all done in V4m152
Hope this helps
Simon