Hi,
As far as the priority is concerned you can set it to do which server in a sequence. i.e. If you are backing up five servers and if you want to backup the Exchange on server 4 first and then go ahead with SQL backup and so on.... you can set the priority when you click on the submit button besides "global options".
As far specifiying which server to be backed up at a specific time that is not possible within the same job. You can only priortise or set the sequence of backup.
bhanu