NEVER EVER attempt to share a Time Machine drive with storing data. The reason being is when the drive fails (yes all HD will fail) you will lose not only your data but it's backup too! Be smart and buy yourself another external HD to use for storing data. What can take up a great deal of space on your computer is media such as photo libraries, movie libraries and music libraries. It is wise to move these libraries (please read Move your Photos library to save space on your Mac - Apple Support) and your Time Machine backup will back that data up too.
When you are shopping for an external HD, also it is wise to spend a few dollars more to buy high quality storage. Many experienced users on these forums prefer the OWC Mercury Elite Pro series due to their efficient cooling which translates into longevity, reasonable pricing and great support. As a general rule stay away from Walmart quality EHDs such as WD, Toshiba and Seagate enclosures. They are designed to be throw away and have poor cooling. The internal HD's are good quality, it is the enclosures they use that are awful.