Can you have more than 1 WP install on same DB ?
 

Just under different DB username?

thx

yes you can use more than 1, you edit the prefix i believe.

Can you explain me, how to do that?

Exactly what killer said, just modify the table prefix, such as wp1_ and wp2_

when you run the install on wp is ask for a table prefix.

http://i45.tinypic.com/2ilbt5k.jpg

I found it, in wp config file, thanks a lot! My DBs at one host are on maximum :) , so it was a bit tricky what to do until you told me :)