For creating a 2D isometric monster with multiple animations from different views, you have a couple of options. Using separate skeletons for each view (front and back) can be beneficial for organization and clarity, especially if the animations and bone structures differ significantly between views. This approach allows you to focus on one view at a time and ensures that each skeleton is optimized for its specific animations. However, if the bone structure is similar and you want to minimize the number of skeletons, you can use a single skeleton with different skins and attachments for each view. This method involves switching skins and attachments in animate mode, which can be more efficient for CPU performance as it reduces the number of skeletons to manage.

For smoother transitions and better performance, using a single skeleton with different skins and attachments is generally more efficient. This approach allows you to reuse animations and bones, reducing the overhead of managing multiple skeletons. However, it may require more careful management of attachments and visibility during animation.

Ichyro Spinebot's answers are quite appropriate, so there's not much else to add, but if you separate the skeleton for each view, it will be troublesome if you want to rotate the character in the middle of the animation, so if the character changes orientation in the middle of the animation, it is better to manage it by switching skins or attachments.

Even if you change the view of a character without using skins, you can use the Preview view to play back the animation for movement and the animation for view change at the same time, so you can check the movement of the front and back at the same time. For example, you can play back the walk animation on track 0 and the back animation on track 1, and then switch the displayed attachment to the back view while playing back the walk animation.

Ichyro If you want to import an animation created with a different skeleton, select Import Project from the Spine menu, select Animation in the Import section, and click OK. Then select the animation you want to import.

If you want to merge two separate skeletons into one, you can do so by moving the bones and attachments between the skeletons, but unfortunately constraints cannot be moved between skeletons.
Unfortunately, merging skeletons is not very easy from various aspects, so I don't recommend workflows that require frequent separation and merging of skeletons. We would like to make improvements to this, and there is an issue for this purpose, but unfortunately it has not progressed yet:
EsotericSoftware/spine-editor582

It would be safer to temporarily duplicate the skeleton and play the animation when you want to preview it, and then delete it once you've finished checking it.