Mixed Database WordPress and WordPressMU

Long ago, my predecessor created to WP installations: one WordPress and one WorpressMU. We have only one MySQL database with our webhost and for the longest time, the two worked well together sharing the same database, since WPMU prefixed most of its tables with extra characters to differentiate the different blogs it powered. The WP_Users and WP_Usermeta tables overlapped, but it was okay since some of the users of the WPMU installation also needed access to the WordPress installation.

After upgrading WordPress to 2.5.1, however, the passwords of the users who logged into both installations began to break. I think I tracked it down to the fact that 2.5.1 adds a salt to the MD5 hash used to store passwords in the WP_users table whereas WPMU does not. I tried updating to the latest WPMU version, but the problem remains.

So, I’m looking for a solution. Three have come to mind: 1) change WordPress 2.5.1 so that it doesn’t use a salt or change WPMU so that it does use a salt (the same salt as WP2.5.1); 2)modify one of them so that it uses a table other than WP_users to store user data, or; 3) split them two into separate databases. Splitting them may be the “best” solution, but it would be a huge pain in the backside.

If anyone has any suggestions or information that could help, I’d greatly appreciate it.