Transparence

I'm working on a show in south of france. 

We have live video, we control differents sources tanks to an Atem. 

I have a computer with Millumin, sending recorded video, link on the atem as a source. 

What i would like to do, is to send recorded video on the live video, so we can see both source, one on the other. 

I have an export project from After effect set for transparence. But when import it in millumin, the background is black. I can control thanks to the atem, to have 50/50 of recorded and live video. 

But i can't remove the black background

Do you if there any effect in Millumin for that ?