ffmpeg filter_complex problem
filgergraph as described can not work, you can not use [alpha] twice, you need to split it:

[0:v]split[bg][key];
[bg]crop=720:576:0:0, format=gbrap,setsar=0:1[rgb];
[key]crop=720:576:720:0,format=gray8,setsar=0:1,scale=720:576,split[alpha2][alpha];
[1:v]scale=720:576,format=yuv420p,setsar=0:1[video];
[alpha]negate[neg_alpha];
[video][neg_alpha]blend=all_mode='multiply':all_opacity=1[masked_video];
[rgb][alpha2]blend=all_mode='multiply':all_opacity=1[masked_3d];
[masked_video][masked_3d]blend=all_mode='addition':all_opacity=1