Windows should allow you to "mirror" monitors which just means theyre displaying the same thing. So if you have a dual output card, or two cards, you can switch between extra desktop space and mirrored.

i also concur with weed.  I have a had two video cards in my machine for dual monitor support in the past, via: ATI Radeon Vivo, and Geforce 2 card.  And have had an ATI Radeon 8500, 2 Matrox cards, an STB with 4 connectors to plug in up to 4 monitors, and a Radeon 9700 Pro.  All had support to just mirror the output, or double the screen capacity.

Ok, I'll have to go for the two-card solution then.  That brings up another question (I will add points if you can help me out on this one too):

I have only 1 AGP slot in the PC and only 1 of both video cards is PCI.  So I'll have to put 1 card in an AGP slot and one in PCI.  The AGP one is a GeForce 256 (has only a video output), while the PCI card is a Voodoo2 (also has a video input, to rerout the video output from a normal videocard into this grapcics accelerator.  A.k.a. the "old" way to do it :)).  So before I try this out, here are my concerns:

1. Will AGP and PCI mix?  For example, I'm thinking about bus speed differences.

2. How will I get the video signals to the Voodoo2?  Normally, I should do this via the input of the card, but that won't work here.

Your answer is yes on #1, AGP and PCI will mix.

#2.  I am not sure a voodoo2 will work for your situation?  As for i have never tried one of those cards for dual monitor support.  It was meant to work for an addon to a 2d video card to add 3d support.

But here is something i found that may be of interest:
http://www.realtimesoft.com/multimon/articles/voodoo2/
says it can be done...but in 2k.  So depending on the OS...and the availability of drivers....it may/may not work.

One thing to think about is that if your cards are not similar...depending on the type of programs/games you will be using that may utilize the dual monitor mirroring or expanding desktop, one card may not be as quick or able to keep up with the other card.  Both cards you listed are older cards.  

What OS are you running?

Also consider the cheapness of the newer video cards that are not necessarily the top of the line.  You may just want to upgrade the voodoo2.  

http://www.newegg.com/app/ViewProduct.asp?submit=manufactory&catalog=289&manufactory=1419&DEPA=1&sortby=14&order=1

Here you can get a Sapphire Radeon 9000 card which will do dual monitor support.  Just need to get an adaptor (may cost 10-20 dollars) from DVI to SVGA.
This comes with drivers for mirroring or expanding the desktop.  The card is $99.00.

and is a better card than the two you have.

Anyway i will add more comments later.  I must go to work.

Umm, weed and wakeup, I think I did point out "The two video card solution also works, as there is a setting for simply echoing the output" in my original post.

wakeup has answered follow question #1, I'm also not sure about the second, but here is my thought:  any new PCI card will be MUCH better than the well-aged Voodoo technology.  I think 3D support is not a big deal anyway.  In two video card setup, the game will take over the primary only.  This is how the wide desktop mode works.  Some experimentation is needed to see if this works with echoed screen mode.  Has anyone here tried that?  Alas, I am down to single screen these days.  But even if it does work even when a fullscreen game takes over, like I already mentioned a new PCI card will run circles around a VooDoo.  TNT2-64 chips are popular in PCI cards; a good choice and definitely don't need a Voodoo addon card.

Increased points.

Thanks, I know what I need to know now :))

I'll post another Q with points for Wakeup for his last answer.  Thanks all!

Ok, I'm not an expert but I had to register as one to offer this solution. I, too, was looking for a solution (mainly software-based, not switch-based or single interface/mutliple user as suggested above) for the dilemma of using the processing power to share my computer throughout my home without having to buy extra PCs. It took me 20 minutes or so to find this information.

Please follow this link:

http://www.thinsoftinc.com/products_betwin_info.html

This is a software-based multiple user interface with a simple hardware solution. Its does require more than 1 video card (as expected) but at least it does not require an expensive switcher and an honest, seperate user interface for up to 5 users.

Please comment on this, any other solutions that you may find, I would like to know more.

Mark Stephens, Nova Scotia, Canada
markyuristephens@hotmail.com

Sorry Magarity, didn't notice your answer. Magarity is right, you need a thin client solution. The link I provided above is one particular solution.