I’m sure others have found great sites that have wallpaper designed especially for dual monitor setups. Does anyone care to share? Thanks in advance!
There’s a lot of cool stuff at Digital Blasphemy. They have a section somewhere with a bunch of images in common two-monitor sizes.
Although he offers some very nice work, I do not have the money to pay for wallpaper. It appears as if all the dual/tri monitor wallpapers are pay only. 
I do not have two monitor. Frankly, I have doubts about the one I have, I think it is sneaking out and seeing other people. However, I have been to KatamariDamacy on the web, too see what the fuss is about, and saw this , which should be right up your alley.
So odd. I was helping my brother on his computer today, and noticed his desktop was the same on both monitors. “I wonder if anyone has designed any wallpapers specifically for dual monitors?” I wondered.
That was maybe 30 minutes ago.
You are not crazy, just so you know. That above posts really exist.
That being said, here is a google search on the topic. I can’t really recomend one over the other, but you may find one you like.
I had the same interest when I used 2 monitors with 1 computer using Windows XP. There were 2 different options: have the same desktop on 2 monitors or have 1 main desktop on monitor A and have a desktop extention on monitor B.
When I had 1 main desktop on monitor A and a desktop extention on monitor B I could move the mouse cursor from monitor A to monitor B seamlessly (left to right.)
If your screen resolution is 1024 X 768 on monitor A and B, your desktop is now 2048 X 764.
You can take two 1024 X 768 pictures and edit them together in the Paint application or find a very wide picture that will be 2048 X 764 (if 1024 X 764 was your beginning resolution.)
It’s fun to make your own!