Thanks, Estel! I tested some things, and yes, both debian and ubuntu uses maemo's /home/user directory when they are mounted... So this indicates the question: what should i do to separate the systems from each other...?

(just one more thing: when i set for example wallpaper in debian from the directory "lxde", of course it does not appear in ubuntu, but when i set ubuntu's wallpaper from "lxde/other", the debian wallpaper disappears; and vice versa. the same thing happens with everything that i change: panel settings, appearence, etc)

Thanks, jm