How to get this transition using ffmpeg? I have bee experimenting with ffmpeg on video transitions , Now am stuck in this transition , I have no where to go ,

I have two videos like this,



at the end of first video i have to split the first video into two from center and reveal the second video like the following, 


any help would be appreciated
Some variant of:

ffmpeg -i FIRST -i SECOND -lavfi "blend=all_expr='if(lte((W/2-sqrt((X-W/2)*(X-W/2)))+N*SW,W/2),A,B)'" OUTPUT