table prefix

i have a mysql database that has two sets of files that have different table prefixes. my problem is when i change my table prefix in my wp-config, either one or the other happens:

1. i cannot access my wp-admin
or…
2. my wp blog doesn’t show my (the correct) wp blog contents (like posts, sidebar, etc.)

so when one happens, the other doesn’t work. when i switch table prefixes, then the other works. (visa versa)

why is that, and how do i fix it so everything works.

confused and sleepy,

alx