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I cannot think of a single reason to do so. That would be like using Photoshop to build a database - a completely inappropriate tool for the job.
Some video IO cards can output matte and fill channels simultaneously, although I haven't noticed the feature advertised much recently. Investigate Blackmagic, Io and Kona cards for a card that has the feature included.
>if so, can after effects also output a live matte for keying that
>animation over another video signal?
Sorry friend, even the thought of it is silly (no offence). Matte and Fill signals in the video world have a completely different meaning than what you do in AE. There is a reason why this segment of the broadcast market is dominated by dedicated systems from Chyron, Harris, Quantel, Orad, Virtus and others and not by After Effects. ;-) In short: You will have to buy one of those dedicated systems or at least a piece of software (there's a software-only version of Inscriber that supposedly can work on some capture cards) and that is going to cost some money. If you wanna have the quality you are used to in AE (full antialiasing, motionblur, blendmodes) it is going to really cost a lot. You should find a video rental company that offers such systems for tradeshows or sports events etc. and fetch a good price with them so you can lend one whenever you need.
I recall 5 or 6 years ago running live graphics for a huge public event from a Mac G4 with a Digital Voodoo card. I was able to play out Quicktimes w/alpha as matte and fill simultaneously from two SDI ports, using the proprietary Voodoo media player that came with the card.
It was quite straight forward, but obviously there are better tools for the job. AE wouldn't be able to output both channels simultaneously, let alone actually play video. But for pre-rendered Quicktime files the card did the job.