TM performs best on its Own Dedicated External Hard Drive,
Mac 101 Time Machine
Time Machine Tutorial
Video on Restore From Time Machine
It is recommended that the TM EHD be at least twice the capacity of the Drive(s) it is intending to back up.
If you connect your current EHD to use as TM backup it will be Erased during the TM setup process.
On both my MBP and iMac I prefer to use external drives partitioned to have a Time Machine backup, clone of the internal drive as a backup-bootable volume and another to act as extra storage. That way there is a drive to boot and restore from and another for mass storage.
Drives are inexpensive now in large capacity from sources such as OWC. Their Mercury Pro Elite in 2 TB is less than $200...hard to beat.
I use a seagate freeagent Go drive to keep a copy of important files. Files that I use with some regularity. I sync the drive every few days to ensure I have the latest. It is a old school method, but works well. I realize that not all things are "backed up" as they would be with a real back up, like time machine.
Can one device drive do both or should I use two different drives?
mb pro 2011, 4GB, 500 GB sata disk
Works fine on the one drive, but to be safer, simply partition the drive into two, make sure the Time Machine partition is around twice the size of your Mac's drive.