I've been looking at these two tags, trying to come up with some reasons as to why we have both of them.
I started by looking at the descriptions:
Ok, so questions about the cinematic universe created by marvel are on topic, we are a Movies and TV site. Answers can reference other media types, however a question about comic-book Hulk would considered off-topic.
We could just merge marvel into marvel-cinematic-universe (or vice-versa, but see the note below), However, as I've already mentioned, we are a Movies and TV site, i.e. the 'cinematic' part of the tag is redundant.
I propose that the tag should be marvel-universe, with the description:
A fictional universe populated with superheroes (and villains), which is the setting of many cinematic films produced by Marvel Studios. Questions should use this tag when asking about interactions that affect the overall universe, not sequences in specific movies.
What do you think?
Note: I'm also averse to having a blanket 'marvel' tag. I think it would get misused, added to questions that are about a particular movie, or a sequence which has no bearing on the overarching universe (like the wiki description above implies).