I'm not a mod - certainly not here, nor in other places. However, I've got some inkling of an idea of what goes on with merges. If a mod wants to correct me on this, I look forward to a better understanding of the gears of the system.
The primary reason to merge is to move the content from one question that is an exact duplicate into another.
Fun Tan Question is an exact duplicate of $ \tan 1^\circ \cdot \tan 2^\circ \cdot \tan 3^\circ \cdots \tan 89^\circ$ and there are distinct answers in each. Merging the two may make sense - to collect the answers in one place where there is one canonical question with its set of answers. This helps so that when next time the question is asked, people find the canonical answer.
In the situation you describe, such that there are two questions that are effectively the same and could be merged one then needs to look at the answers themselves. The primary guiding metric should always be "is this answer indicative of the desired quality at the site?" and "would the resulting question's merged in answers be new and distinct from the existing ones?" If the answers to both of those are 'yes', then consider doing a merge.
If the existing answer already has a complete answer and then some hint answers, and the merge would bring in other hint answers that are more or less the same as the existing ones it may be important to take a hard look at those answers and ask "do we really want to encourage the resulting question will have more hint answers that have been up voted from their previous incarnation and give the appearance that it is ok and proper to pile on a dozen 'hints' to a single question as they are all up voted?"
There is an edge case to look at in here also - that one person has an answer in both questions. In such a case, it may be more appropriate to give that user a link to the deleted answer (in comments) and allow them to hand merge the distinct material.
The thing that dups and merges do is help with SEO. More titles that point to the same question, the better. When a person hits a dup with no answers (merge or dup) from google or another outside source, that person will get redirected to the question with answers.
This brings up the question of "does undeleting the question, to close dup it, and merge it, and clean up the answers in the resulting merge improve the SEO of the site?"
If its a question where the title is "Fun Tan Question" it may not. That particular string isn't something I'd expect someone to search for when looking for an answer to the problem.
It may be better to leave a comment on the undeleted answer pointing to the deleted answer (that you have a link to) and ask if there is material from that answer that would be appropriate to bring into this answer.
Something of a note here that it may be more productive to work at improving the quality of the question and title so that it does get better SEO itself. With that tan question linked above, it doesn't show up on the first three pages of the google search tan 1 tan 2 tan 3 ... tan 89. This may be because it has mathjax in the title (its 'real' title is:
tan-1-circ-cdot-tan-2-circ-cdot-tan-3-circ-cdots-tan-89-circ) and this may be hampering google's search. Just something that you might want to bring up with the SE devs to see if there's a better approach to this.