I'm trying to find out if it is feasible to build a box that is
attached
to a single rooftop antannae but with multiple tuner cards/chips in it.
I'm trying to record multiple shows over-the-air(onto local disks)
simultaneously.

Sure. You'll want to have some sort of 75-ohm splitter, to divide the
signal from the antenna and provide isolation between the tuners.

Depending on the strength of the incoming signal, the number of ways
you wish to split it, and the sensitivity of the TV-tuner cards, you
may need an RF amplifier (either at the antenna, or as part of the
splitter).

How many simultanous recording operations you can do is going to
depend on the tuner/compressor/capture cards, the PCI and disk
bandwidth of your hardware, the sophisitication of your operating
system and application, etc.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!