The difference in brightness might not be a big problem for you. I've used a simple VGA Y-lead for years as a basic test jig when doing monitor repairs. When you connect the second monitor you can see the first one dim a bit. Another thing I've had happen, but rarely, is for the lower signal to make a monitor lose the signal completely, because it doesn't sense the lower input level as a valid signal.
Remember that both screens will be displaying an image of the same resolution, so the image on the TV will probably not look right, unless the monitor has the same aspect ratio. Also depending on their respective native resolutions and how the TV scaler circuit handles inputs, you might have to use a resolution the TV wants, and put up with how that looks on your monitor.
If you use a Y-lead, don't leave a lead on the second arm that's not connected to anything - it won't do any harm but the image will go blurry. If you don't realise what's happening, you can think your original monitor or video card has gone faulty.
Edited by Platypus, 14 July 2010 - 10:13 PM.