Two internal drives - one not showing in 'Startup Disk'

[Becca]

Mac People: Help!

As the title indicates, I have two internal drives installed in my Macbook Pro (from 2012). The most recent installation replaced the DVD drive. Up till now I have done everything right: the second Samsung SSD drive was installed and setup by a professional and in the beginning it was easy to use both drives back and forth. The whole point of it was to downgrade the new drive to Mavericks so I could run Pro Tools 10 and Waves on it. That worked fine as well, except that when I wanted to start up on my main drive (with El Capital installed) the Disk Utility popped up and I didn't know what to do other than back up everything and delete my main drive.
Now I just don't understand why the main drive is completely visible in Finder, Disk Utility, Time Machine and everywhere else but in 'Startup Disk' so I can actually use it (I need El Capitan to open school documents with the newest version of Word).
So the dream scenario is to use two drives; one with Mavericks and one with El Capitan.

Hope somebody can help

 

[Spawn_Dooley]

The drive named 'Jimi' appears to be a Time Machine drive?

I can see that 'Freddie' has Mavericks installed & is the primary startup drive but I can't see the El Capitan drive in your images ...

When you boot up holding the Option key after the Startup Chime, does the El Capitan drive show in the Startup Manager?