I personally use two skeletons - one for front side and one for back side. For my characters at least, the skeletons for the front and back sides are too different to be shared (front side has all of the bones for the face, different bone structures for hair, etc). If you have super simplified characters (torso/arms/legs/head) then maybe you could share the same skeleton - but you'll likely still be making totally separate animations for the front and back sides anyway (like, you'd still have to make a 'running' animation for both the front and back side).

For what it's worth, I initially tried using the same skeleton for the front and back side (and used different Skins for the front and back), and it ended up wasting more time then it saved, so I switched to two separate skeletons.

Hello!
You can successfully rig any direction inside the same skeleton if you want:

A series of streams where this character was rigged showing the complete process can be found here, along with the downloadable project: Spine: Twitch

The first video showing the rigging is here:

I suggest navigating the videos using the chapters in the video descriptions, as they can be quite long given that they showed rigging live.

The technique revolves around reusing the same bones for the different directions, storing then in a turnaround animation, and having in setup pose a neutral pose.
If you only have isometric directions for your character, you could rig the front first, which can be rich in details, and then reuse what bones you can for the back of the character.