anything from Photo Shop Pro 5.0 and newer has Layers & Channels built-in.
AiST Media Mania TV 3.0, Avid Studio 4, AiST Extreme 4.0, Asymetrics Video Producer 5.0,
Magix Pro 2001, Sony Vegas Platinum 8 and Adobe Premiere 8 all have these same features.
Coral started the trimming and dissolve thingy long ago. They even made a plug-in
for it called, "Knockout v1.0."
The concept was simple. Isolate the character/object you want. Paste it wherever. Repeat as needed.
seems simple enough. once you have isolated the object, position it with an Alpha Channel
to hide the rest of the object. place in location, press Enter.
The only limit you have are the number of layers your editor can handle.
if you do it wrong, it will look like this ...
The beauty of doing things in Layers are, "you tell it who is running behind which image."
Each Layer can have its own Chroma-Key. This makes superimposing simple.
To create a Mask for those editors which do not have Auto-Masking, each frame must be matched
picture-to-picture, frame-to-frame the old fashioned way. Then Alpha-Channeled to Isolate the
object and finally pre-positioned in place for the Mask to Work! Call the Chroma-Key and on to
the next frame. Cost-wise, this savings is not worth the frustration. however, if you don't need it,