Hello hutro and welcome to the forum 🙂
REI3 currently can only display/navigate to one form (and sub forms, etc.) within the same browser tab. If you need to edit multiple records at the same time, you would open a second browser tab and then have both tabs visible at the same time.
It is possible to have "multiple tabs within one browser tab" - we had that feature in a much older version of the platform (REI1 if I remember correctly). And while it is useful in some contexts, it makes navigation inside the browser more complicated, especially when using the back/previous action. Because having multiple browser tabs open side-by-side is easy and keeps browser navigation understandable, we decided to not implement this feature.