Correct with the bright\dark answer if we are talking all one clip here.
But it looks like an effect was applied to add the second image to the clip, and then the dissolve was added to a single clip. Dissolving in (with a white background) may not be the way to go in this instance.
Alternatively, try interchanging the blend modes of the effect with multiply, screen, difference, luminance and color. These are the base blend modes that, when done in order, will give you a blend then it’s opposite. If none of them work, try the next method.
Last alternative:
Set clip A on a track above clip b. Let them overlap time, i.e., let b slide into the end of A, and fade (Dip) a to black. One of these will make B will come in (with no added effect) darks first, with a cross dissolve look, since it is playing underneath clip a.
You can reverse this by moving clip B to another track, overlapping A, and placing a Dip to white on clip b where it overlaps.
If you want to strengthen this effect:
if you move clip A, place a dip to white on b, and the dissolve will be sharper.
if you move clip b, place a dip to white on clip A that is .75 to .5 the length of the one on clip b, so there will be a faster drop on A, while b is shifting in. Since the Blank of the DIP will match the background of clip B, you won’t notice changes in the background, and the darker areas will show up together onto that background.
IF they have two different sound tracks, you can fix that with keying on b, keying on a, or more effectively on both, to create a sound dissolve. IF you have one sound track and want to keep it that way, unlink the clip you move from it’s sound, then move the video only to another track.