SQL Backup's timeline feature is there to warn you about things like this - because SQL Server does not allow you to run two backups on the same database at the same time, it is telling you that you are going to have problems. But it does not matter how many databases you are backing up. It's about two jobs on the same DB (or it should be).
The solution is to stagger the backups or to do them less frequently. For instance if you are trying to log ship every five minutes and the log backups take ten minutes, the backup jobs can fail.
I am not aware of the editing problem you have described - I am only aware that if you try to edit a job by hand in SSMS, it can break the parser in a way that you can no longer edit the job using SQL Backup.