It works. I have an internal SSD for Mac OS X and an internal HDD for data storage in my 2009 MBP and I can use them at the same time.
Why do you think, it shouldn't work?
A Mac Pro can have up to four drives (even more when wanted) and they can be accessed at the same time too.
And an optical disk drive (ODD) is just another drive, you can access it at the same time as you can access any other drive.