I back up all three system databases to a single file and don't have a problem with that, otherwise each DB backs up to its own file nightly with overwrite. I do a noon incremental of our ERP system to the same file that I backed up to the previous night, but it's already been backed up by Tivoli at that point, but I think the risk is negligible as I'm also backing up the transaction log every 10 minutes. Unfortunately I don't have enough disk space to maintain multiple copies of our ERP system online; normally it backs up to our SAN box that has 6 TB of space, but that's not working at the moment so I'm dependent on Tivoli right now.