You have to have another Time Machine configuration in which the external backup drive is selected as the Time Machine backup drive and the other external is the source drive. It's my understanding that Time Machine can now have multiple configurations in the same setup, but I don't use Time Machine so I've never checked that out.
I use Carbon Copy Cloner for my backups. It allows a simple setup for each desired backup. Each setup can be scheduled for different times of day, frequency, and day of the week. Unlike you, I have one backup drive for each drive I backup. Putting more than one backup on the same drive puts both backups at risk should the drive become corrupted or fail. Each backup occurs at half-hour intervals on a backup day. For my system, 30 minutes is ample for each backup. Like you, I do keep multiple backups in case a drive fails or becomes corrupted. You can never have too many backups.
I'm not sure if there is much more I can tell you that you don't already know. I am not sure that Time Machine can now handle multiple backup configurations, but I seem to remember that being part of Time Machine's major rewrite several years back.
In the Time Machine preferences, you select your first backup drive. Configure your first backup. Now, from the list of disks available choose the next backup disk. You are prompted if you wish to change the backup disk or to also use the newly chosen disk. You apparently repeat this until you have selected each desired backup disk. Then make the second part of each Target - the Source disks. Remember that Time Machine sets a Source disk by excluding the disks you don't want for this backup configuration.
Maybe this will help get you started.
