Let's say your resolution on the monitors is 1024x768. Create a new image with 2048x768 resolution, and copy & paste the wallpapers you want, then save that image and use it as your background image tiled (not center). If you want to stretch one image for both monitors, just stretch it to 2048x768 resolution and once again tile (not center).
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Multi-monitor desktop wallpapers
Use the wallpaper feature of UltraMon™ to create desktop wallpapers with a different background on each monitor, or a single background stretched across all monitors.
Other options:
Active Desktop: go to Control Panel, open Display and switch to the Web tab. Make sure the 'Show Web content' box is checked. Click 'New...' to add your image(s). You can stretch a single image across the whole desktop, or use a different image on each monitor.
http://www.realtimesoft.com/multimon/faq.asp
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