I would echo Psy053's comments. I've found that full backups with competing products are not a problem, but incrementals are another story.
By default, CommVault uses the Micro$oft Windoze change journal to select files for an incremental/differential backup (am assuming we're talking Windoze here - all bets are off for Unix). That's fine, BUT, some competing products which use the archive bit for file selection clear that bit after backup. Now you'd think that with CommVault using the Windoze change journal to select files for incros and another product using a file's archive bit, they wouldn't impact on each other. Bzzzzt! Wrong! The action of clearing a file's archive bit is a "change" as far as Windoze is concerned, so results in the file being put into the change journal. Along comes poor CommVault and sh*ts itself when it sees just how much it has to backup as part of that incremental. I can just hear it now: "What the? How come they have changed 45 million files and I have 45 million files to backup for tonight's incremental?". The answer is: another backup product cleared the archive bits of those files resulting in them all being put into the change journal.
In summary, competing backup products are OK to run side by side, provided you don't mind HUGE incremental backups and aren't worried about all the I/O 2 products will generate. But best stick to one eh?