I think there is some overlap between the two and they should be separate, with some better wording and usage rules in their respective tag wikis and excerpts.
The off-topic tag should refer to discussions on off-topic subjects specifically. and the tag excerpt probably need to be better worded. Examples of use may be why something might be considered off-topic or why something should be considered off-topic. Such as is this topic in scope of what the site and users can reasonably handle.
While we have had questions in the past related to law and copyright in terms of anime, such discussions are generally considered off-topics as we are not experts in the area and cannot offer adequate advice. Same thing with creating our own anime/manga as the process varies by country and publisher. Also reading/watching recommendations, as they tend of be very subjective, different people have different tastes.
The allowed-topic tag should cover discussion on what we allow and don't allow, specifically. Like links to 3rd party scanlation sites that may violate copyright, or graphic/pornographic content. For an example, the discussion on the topic of "hentai" is allowed, but graphic content from hentai is not allowed (with restrictions) unless it it required to prove a specific point (i.e., not how it for the) sake of showing it).
The acceptable-questions tag on meta should most likely be synonymized with allowed-topic.