Advice wanted: Azure AD Join a PC with own O365 when working as an interim for clients?

Most of my time I spend on connecting users and their computers to an O365 environment. When possible (when they have a computer that runs Windows 10 Pro or I can convince them to upgrade to it) I will set up their computer Azure AD Joined, so they can sign in with their organizational account. This way their whole user experience is involved around Office 365. Everything will find their O365 credentials automatically (I know this can also be achieved by connecting it to a local or Microsoft sign in account), but my main positive effect is that from start to end they just need to remember 1 account from Microsoft.

Now, at this moment I have two seperate prospects who want an O365 subscription for their own company. Both these clients will do a lot of work for other companies:

- One will work as an Interim HR Business Partner, doing 2-3 days a week work for her client for 6months to a year.

- One will work as a Virtual Assistant. Don't know yet how she will connect to her clients.

I happen to know that the HR Business Partner has ordered an HP laptop with Windows 10 PRO preinstalled, so at first I was happy: Azure AD Domain Join, etc. 🙂 But then I started doubting... is this the best way to configure her machine.... because, when she opens Edge and browses to Office.com, it will automatically sign in with her own company organizational account.

But what if her client also uses Office 365 and she does work for them as an interim 2-3 whole days a week. Then everytime she wants to open their O365, she has to log of her own??? 

Or are there easy ways to switch between environments?

What do you suggest to configure her own machine?