Managing Multiple Tenants from one User

[creativegap]

Hi all,

Earlier this year I implemented Office 365 for a small company (from now on referred to as TenantA) I was working for at the time, however, I have recently left said company and they have asked that I continue administrating their Office 365 for Business tenant. 

In addition to this, I have my own Office 365 for Business tenant (from now on referred to as TenantB) which I use for my own freelance business. 

Other than through the Microsoft Partner Network, is there any way to add user myself, logged in within TenantB, as a 'full-access' global administator for TenantA, essentially granting them permissions to access both tenants, and administer all properties within each?

At the moment I log in to TenantB on Microsoft Edge Beta, and TenantA on Chrome, however I'm hoping there's a better method. 

EDIT: Just to expand on the above, I have gone from 'employed' to a 'consultant' (as a graphic designer/event manager with the company known as TenantA) therefore will still need access to Teams, OneDrive, and Sharepoint files, but would like to do this from a single user on TenantB with Global Administrator rights to TenantA.

[#2]

Generally speaking, Guest users can be assigned admin roles, however I haven't seen any document saying that this is a supported scenario. Moreover, the best practice is to use a separate account for performing those admin tasks, and do your day to day activities with an unprivileged one (or enable PIM). 

[#3]

@Vasil Michev Yeah, I definitely got into bad habits with using an Admin account for everything. The company was a small team of 8 users, no IT department, and technophobes, so I found it more sensical to use the Admin account for everything given the regularity of having to assist someone given that we were migrating from Google Drive/Slack/etc to the Office/Teams/Sharepoint ecosystem alongside my 'actual' role as designer/marketer. 

The main issue is I got used to doing things wrong, and being able to just go straight into office.com and administer their system; I'm now an 'external freelancer', now means I have to sign out of my own Office tenant.