(I don't know much about flash, but..)
Here: Slack 13.0 (32bit), FireFox 3.6, flash "10,0,32,18"
I do not experience the problem you describe. I can play two or more flash videos at once.
ls -al /usr/lib/mozilla/plugins/libflashplayer.so
-rwxr-xr-x 1 root root 10278616 2009-07-17 21:10 /usr/lib/mozilla/plugins/libflashplayer.so*
It might be good for you to post what version of slack, firefox, and flash you use.
Can you duplicate the problem in seamonkey?