[ DISCLAIMER: This is a bit if a lengthy post - I may tend to over explain things; Also the information herein is based solely on the research I have personally done and may contain some fallacies that I am unaware of.]
I'm not sure if you've got this resolved or not at this point, but I figure I'll add my two cents to this in case it can still help someone.
First and foremost; I've made a couple posts around the forum here about similar topics related to combining databases for multiple servers - specifically on the same physical server.
(See the following links for reference.)
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How to run multiple 'realms' on the same physical server
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My past configuration of WoW Emulators using the same Auth server.
Long story short; it is very much possible to combine certain databases when hosting multiple servers, if you make sure none of them try to read/write conflicting data in the same tables.
When you really look into it, MySQL is very fungible in what you can do regarding databases. It is, in fact, possible to have multiple applications (such as a WoW Auth. server and Minecraft permissions plugin (as an arbitrary example)) that point to the same database but reference completely different tables within it, never touching each others' information. That said, there are certainly scenarios where conflicts between multiple programs trying to manipulate the same database will end very poorly.
In our immediately relevant use case, the three relevant databases for any particular WoW Emu. server have unique functionality when introduced to other, similar game servers:
- Firstly; the 'World' database - which contains all the information about NPC and item statistics as well as some localization of text for different languages, among other things. Given that this is mostly referenced for information instead being written to, it (from my experience) is pretty safe to allow multiple server instances to point at it at the same time without any conflicts. This will only work, however, for game servers running the same patch version of WoW (3.3.5a, 4.3.4, 5.4.8, etc.) as the content from each expansion pack is drastically different and the server(s) couldn't parse the disparate information. It is also not a possibility to, as mentioned above, combine multiple servers' 'World' databases into a single unified one as they do share the same tables and will thusly also encounter the aforementioned conflicts. With that said, using a single 'World' database when trying to run multiple similar/identical servers can be very useful to reduce CPU and RAM usage and simplify updates & patching between them.
- The second 'Character' database is pretty straightforward - This is what store all the data unique to a server (Characters, inventories, AH listings, raid/dungeon timers, etc.) and shouldn't ever have have more than one server pointing to it.
(A possible exception to this would be if you were testing different server .exe programs and want to compare them against each other using the exact same environment.)
- Lastly, what this thread is really about, the 'Authentication' database. First a bit of background: From all the information I've found scattered around I can say with relative confidence that every WoW server emulator (which I could find a source for) is at its core, built upon MaNGOS -
the Massive Network Game Object Server (which, itself, technically isn't exclusive to WoW and can be used to create emulations of other MMOs' server-side). What this means is that the root of 'all' (as far as I've discovered) WoW emulators share the same method for storing and authenticating game accounts. Therefor, in theory, any WoW server emulator (and matching client) should be able to point at a singular authentication server & database (and vice versa) allowing user connections and redirection, regardless of which version* of WoW they are connecting from. ( I have not testing any version above 7.3.5 yet to see if this holds true for more recent updates as well.)
P.S.
Despite sharing a common base for their authentication, many server Cores / .EXEs have additional functions added in, such as a cash shop, VIP ranks, ticketing queues, and other account-based systems that use additional non-standard tables in the 'Auth' database to keep track of that information. If you are wanting to incorporate any of these such specialized 'Cores', just make sure to add their additional table(s) to your database beforehand.
Tl;dr -
You can totally use the same authentication server / database as a unified login for a variety of WoW server emulators, so long as you don't try anything too extreme.