How do I work with two PowerPoint Presentations simultaneously?

in 1 Screen? or what?

When you are ready to collaborate on a PowerPoint presentation, open it and click the Share button in the upper right corner of the ribbon. In the Invite Users field, enter the email address of the user with whom you want to share the presentation. If you have already saved his contact information, just enter the name. From the drop-down list, you can choose whether invited users can make changes to the file or just view it. Click the Share button. If the file is already saved in the cloud, invitations will be sent to users via email.

Answers to many questions can be found in social networks, as well as to pump your account. How to do this can be viewed here

In order to toggle between two PowerPoint presentations, you have to follow these steps:

1) Click on the File option
2) Choose Open or click Ctrl + O
3) Select the presentation you want to open.
When more than one presentation is open, you can toggle among them by selecting the presentation you want to see from the taskbar in Windows.