Here is the Deal. If you are going to use the same external to store files you don't want on your internal drive anymore,, IE you are going to delete them from the internal drive, then you need 2 backups of those files. Whether one is on the same external as your TM backups doesn't really matter but it is best to make one partition for TM and another for the manually backed up files.
If those manually backed up files are still ALWAYS going to be on your internal drive then all you need is one backup of them. Doubtful that both internal and external drives would fail at the same time, but that can happen also.
As long as you have 2 copies of the files you should be fairly safe.
I have one 2TB external partitioned into 3 drives. One for TM, one for backups (along with multiple other computer in my home that has all those same files on them) and one partition for a Clone of my OS X system.