Complete merge of tenents
If a company is taken over by a company, it should done also with tenants in Office365. This means that all functionality (Exchange, SharePoint, Skype, CRM, EMS, Accounts / AAD, settings, domains, etc.) should merge into one tenant. Also Manageable from one console / tenant. One idea is to do this in stages.
or add multi tenant/multi domain mangement capabilities specific to every user depending on his email domain name