YES, a user can be in multiple companies with the same user account.

Please refer below image as an example. (Refer Image 1)

After sign in to your Orangescrum account, select one Company to get inside. If you are already signed in to one company and need to change your company, then Navigate to User Profile Icon > Launchpad (Refer Image 2)

(Image 1)                                                                         (Image 2)