There are basically 3 options here which I'll try and outline below:
This is always a tough one. Analytics makes it fairly easy from my understanding. You should be able to move all reports suites from Company B into Company A or visa versa. A global report suite can make things a bit complicated unless they happen to share your reporting schema (eVar 1 = Loyalty Level, Prop3 = PageName, ect...). If they don't match your schema, you can still move the report suite, but reporting within a global report suite is going to be limited by the data. For example, the eVar1 report is going to pull back data from both sites and it might not make much sense to compare those items together. You'd also need to look at things like MCID being deployed vs not and tracking server deployments to see if you can start de-duplicating your visitors.
As for AAM and Target, these solutions can only have a single instance under each Experience Cloud company, I don't believe there is a way to move any activities or data across clouds to merge with a different company.
User management is limited across the entire cloud since an Admin over a specific product or a system admin would have access to everything regardless of brand.
2) Start Fresh
I have talked to some companies that find it easier to start a whole new Analytics company and report suites and have a clean cutoff/deployment date. Doing this allows you to have reports from the historical data and new reports that go off the post merger data. This is often chosen because it is the cleanest way to ensure the data match and gives you the most accurate visitor counts.
The same user management principals apply to this option as the previous option.
3) Remain Independent
The third option, is to maintain completely separate deployments and instances based on brands. This gives you the most flexibility as far as permissions and data structure, but it limits your cross brand marketing opportunities and reporting.
Overall, there isn't really a recommended best practice for merging or separating customers. We can tell you the pros and cons of each and you'll need to decide what makes since for your business. If you have more questions, please post them here as I think this is a topic that many of our customers would benefit from reading.
Does my previous reply answer your question? If not, is there something I can clarify?