the apps on stable version are not the same as on testing, different versions
each config file stored inside the /home could be modified by the different apps and give you problems when you log in to the older stable version
i personally wouldn't share configs, only files on a folder between the installs
also, if you have lots of important things on one pc, i wouldn't use it for testing and for stable, too much at risk
pclinux is popular for being stable, even on the TR versions but i wouldn't risk my personal files this way, as always, keep backups of your important files, no matter the os you use