By today, We don’t have the functionality available in the Microsoft teams app for login with multiple work/school teams accounts in the same time and switch to one of the accounts needed. This feature got several user voices (~34000 +) so far, and Microsoft added this request in their office 365 Roadmap.
For now, you can add one of your regular accounts and your personal account only, and adding multiple ORG tenant accounts features will be coming soon. you can refer to the roadmap ID – 68845.
Even this feature is not available yet, many users are getting confused with “Teams guest accounts” and “Regular accounts” and they believe that it’s working for them, and others could not make it worked if trying to add real multiple regular accounts (Work/School). You need to understand that the regular accounts and guest account are different. Guest account has limited functionality and you would be treated as the guest in a tenant for which you have guest access (someone could invite you as a guest).
See the below picture, where I have added multiple Teams account with one regular and remains guest and personal accounts. The disabled one is regular and others are guest/personal. You can also see the mail envelop icon which shows that you have got the invite from these domain users and you need to accept and register it.
1-Login to Incognito (Private) Browser
you can login to one regular account into the Microsoft Teams desktop app (electron) and the other one to the browser in incognito/Private mode (depend upon the browser you use).
2-Portals (A third party application)
MVP James Cussen from Melbourne Australia did a great JOB and has developed this tool call portal which you can use in such a situation as a workaround. This tool works perfectly fine while you need to access multiple Office 365 app s guest or regular accounts. I tested it and find it very useful. You can get more details and download the tool from here.
There are some caveats with this tool as per my testing, it does NOT let you connect teams (other apps also) if your tenant enabled with device compliance to meet the secure access policy, even if you have a chrome extension installed on it. I did not find any fix for it.
I hope the above information will help you to differentiate with guest and regular access and find a way to connect multiple office 365 tenants.