I assume usual software would have trouble to provide a solution which is both simple and fast at the same time for these reasons:
- Would you really want the background? It's empty wasted space - no matter whether it's white or transparent.
- What windows do you want to capture? You will always have to manually select them before capturing. That takes time.
I'd go with a command in terminal:
screencapture -aCW ~/Desktop/1.png; screencapture -aCW ~/Desktop/2.png
The typing may may not be fast, but capturing the pictures is.
This way you can quickly capture two windows of your choice in 'real time'.
You basically skip typing ⇧+⌘+4+space both times.
BTW: This way you can manually choose the image format.