The first is:

    One paragraph.

    Followed by another.

The second is two bold letters, one after another in a single word.

However if the html is "an application" more than it is "a document" - a b-tag with two letters, might be meaningfully different from two b-tags in sequence (for example with css:)

   b { display: block }

So, I'd say as a fragment two bold tags might be mergable - but not in the general case?

Ed: ie if diffing input from a html input field (rich editor) merging bold tags would probably be what you want - when the first edit bolds first letter, and second edit bolds second letter.