Is it possible to have ckeditor toggle between two style rules?

There's a small detail in your question which perhaps makes it hard to solve :). This is - you want to switch between two styles. It is very easy to apply and remove one style, but not so easy to toggle between styles.

If it is enough for you to be able to apply/remove style using button, then check the basicstyles plugin: https://github.com/ckeditor/ckeditor-dev/blob/master/plugins/basicstyles/plugin.js. As you can see it's trivial.

However, toggling is more complicated. I think that what you need to do is:

• Create two styles - one for bold, one for normal.
• Create a button (like here - https://github.com/ckeditor/ckeditor-dev/blob/master/plugins/basicstyles/plugin.js#L36)
• Create a one command which:
  • will be refreshed like here https://github.com/ckeditor/ckeditor-dev/blob/master/plugins/basicstyles/plugin.js#L26-L28 for both styles,
  • will work like this one https://github.com/ckeditor/ckeditor-dev/blob/master/core/style.js#L1421-L1440
  • on exec it should first remove one style and then apply the second style - this way you'll avoid having two spans applying their styles, because AFAIR CKEditor is not able to figure out that those two styles are their opposites.

Anyway... it depends what effect will satisfy you. It should be possible to achieve something quickly, but polishing may require some effort.