I think it depends.
In general I would rather recommend separate skeletons for front and back. If you put front and back on one skeleton, there is no general setup pose. You will always have to copy a manual setup pose on the first frame of each new animation.
Contrary to this, I personally use just one skeleton for multiple directions. But it's just due to the fact that I need isolated animations of the lower and upper body for different directions at the same time. That's quite a complex thing and I'm not sure if this is what you want to achieve.
Otherwise, I would go for separate skeletons.
With some trial and error you will quickly find out what suits best for you.