Probably some stuff in /tmp you can get rid of - it seems to be on /. If you've never cleaned it out, there may be a *lot*. Can also use find and du and other tools to check large space-wasters. Remove unneeded packages. And so on. Not sure what you mean by having two roots and I'm not sure what I'd want to hive off other than /tmp, given that /usr, /var, and /home are already off. (I usually do /tmp, /var, /home, and /usr/local.)
That root partition seems kind of small anyway. If you have another large partition, you could try *switching* roots, yeah.
-- Actually, what am I thinking of? If you've got all of /usr then the root partition should be plenty. You must have *something* wasting space.
(In mine, like I say, only /usr/local where I install my own is off - /usr itself, is on my /, which means I need a couple gigs to have breathing room.)
-- Okay, I'm going to give up now. Last edit. But when I say 'switch roots' I meant moving your current root to a larger empty partition. Just wanted to be clear about that, since you say you have a Slack 10 as well. I don't mean switch roots with *that*.