multiple video sources mixing

I am new to video editing so I am not sure of the correct terms.
I cannot seem to find a manual mixing control. I want to dissolve halfway between two sources and stop. I also want to perform this with three sources.
I want to see all 2 or 3 sources at the same time on top of each other. I cannot find a way to set the opacity to 50% on each source or 33.3% on three sources.

Can you point me in the right direction on this?

It seems that if I had a manual mixing board I could just leave the dissolve control half way between the start and the end of the dissolve.

this is actually not possible in BoinxTV.

Thanks. I figured out how to perform that type of mixing using iMovie.