Such a configuration is not supported. This does not mean it will not work, but it is not guaranteed to work reliably.
Considering the "mission profile" for a backup strategy, you should consider this an unacceptable application for Time Machine.
Read Apple Support Communities contributor Pondini's Time Machine FAQ discussion regarding this subject here:
Scroll down to the pink box.
There are other, significant concerns regarding sharing a Time Machine volume with other content. Time Machine needs to have control of the whole volume.
More info here: Mac Basics: Time Machine