Yes, that's what apps focusing on synchronization of jira instances are for. But through "simulating" that. I mean jira_A has its users (A.1, A.2, ...) eand jira_B has its own (B.1, B.2, ...).
Of course you can setup a LDAP server so every user will login both in jira_A and jira_B using the very same credentials, managed by the LDAP server.
What you can't do is having 2 different 10-user-tier instances and have them act as one 25-user-tier instance. I believe the reason is pretty clear if you try to walk a mile in atlassian's shoes.
So, to summarize. It can be simulated, very well simulated in fact. For the end user can be transparent but this is not simply a configuration issue in vanila jira, you'll need aditional (paid) apps.
Consider the cost/benefit of pursuing this instead of upgrading your tier license and make a decision. I've tried some of this sync options in the past for puntual needs I had and they seemed ok to me.
I hope it helps.
welcome to the Atlassian Community!
If I understood you correctly you probably had two sites under two organisations and transferred one site into your organisation so it now holds more than one site, for example you did according to: https://confluence.atlassian.com/cloud/transfer-products-to-another-organization-967318518.html
Although this does not explain why "Site B" box in your sketch is empty I assume this is the case, if not, please correct us.
If so, you can use admin.atlassian.com to configure User Management as needed.
In case it is unclear you could add a screenshot which sites and organisation(s) are currently installed.
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