The Problem is I doubt the height of the 23" is the exact same as the 17". Despite the resolution, the screens have to be the EXACT same height for it really work and not look stupid. I tried playing with a 2x1 setup with my old 17" and my 21" when I got it, and of course, the 21" is higher than 17", so even though I got it working by setting to a common resolution, playing a game and moving my mouse across the top of the 21", if I kept going right, it would "jump" down an inch and a half to come to the top of the 17".
You should draw your monitors more to scale to see what I mean if you don't get it.
The monitor are actually around 1/4" different in height which isn't bad, I have 2 connected right now (the 23" and one 17") and they are no jump between the 2 when moving windows around, it's all lined up. (I lined the top of the screen in the config panel since the 17" is just a little smaller)
This is what it looks like right now with 2 monitors:http://img31.imageshack.us/img31/8730/480b.jpg
I didn't align them very well, hiding the bezel a bit, but when am facing it I have about 3/4" bezel. For about $20 for the adaptor, might be worth a try but wanted to make sure which of the 2 possibilities above might happen