Can OS X automatically duplicate/backup files after use?

I know there are some wizards out there that either know how to do this or that it can't be done, so here we go:

I have an external drive I use for Time machine purposes, so I do regular backups. As a developer, the loss of my source files would be devastating. I was thinking it would be very comforting to, in addition to my Time Machine backup, I could copy all of my source to another disk, say an SD card or thumb drive.

Is there a way to automate this duplication of data? Some sort of trick to run in terminal or something?

Mirroring? I've never done this and wonder what solutions exist, besides the obvious: manually copy folder contents from HDD to SD card periodically.

Thanks in advance!