Technically you can but it is not a very good idea at all. The reason it is a flawed idea is if the HD fails (not really if but when) you will have defeated the whole idea of backup. A failure means you will lose the data files you stored on it and it's backup! A good backup plan has at least 2 forms of backup because backups can fail too. I would strongly recommend getting at least one additional external HD and use it for storing the data files you wanted off the internal HD and use the second for a Time Machine backup drive.
Personally I have 3 external HD's attached to my computer. Disk 1 is my Time Machine backup, Disk 2 is a Bootable Clone of my internal HD and Disk 3 stores my music, photography and movie libraries.
Please read these articles, they will discuss different backup strategies and I think you will see that each suggest redundant backups.
I keep both on my home external and have never had an issue. But I do also keep a 1TB portable drive with just a backup on it that is always at hand when I am working away. Different people have different methods but it all boils down to one important factor, Always have a current backup at hand, because a disc can up and die at any time.
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.