I assumed both subdomains are being hosted on the same phsical server (does not necessarily have to be the same webserver). If they are being run on separate machines, it gets more compicated - you could use NFS for example.
Or you could use a standalone login check (that's what I would do).
Or you could take the cookies and pass it (through a GET request for example) to the other server, process it and return if it's authorized or not).
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