Copy this into a blank video processor preset and hit Ctrl-S to save it. Put it on which track you've got set up to be the top layer. I can't claim to understand the code itself. I just hacked this out of the Image Overlay preset, took out all the user controls and hard coded it to invert the one image and then add it to the other.
Code:
//image subtract
img1=input_track(0);
img2=0;
!project_wh_valid && input_info(img1,w,h) ? ( project_w=w; project_h=h; );
gfx_blit(img1,0);
img2 != img1 ? (
gfx_a = -1;
gfx_mode = 1;
oldcs=colorspace;
use_srca ? colorspace='RGBA';
z = 10^(zoom/10);
input_info(img2,sw,sh);
dw = (project_w*z)|0;
dh = (sh*project_w/sw*z)|0;
x = xoffs<-1 ? (1+xoffs)*dw : xoffs > 1 ? project_w+(xoffs-2)*dw : ((xoffs+1)*(project_w-dw))*0.5;
y = yoffs<-1 ? (1+yoffs)*dh : yoffs > 1 ? project_h+(yoffs-2)*dh : ((yoffs+1)*(project_h-dh))*0.5;
gfx_blit(img2,0, x|0,y|0,dw,dh);
restore_cs && oldcs != colorspace ? ( colorspace=oldcs; gfx_fillrect(0,0,0,0); );
);