How to see multiple Plans at the same time?
There is no way to see multiple Plan timelines side by side. However, you can use Teams view (Team timeline) and filters to look at assigned tasks from multiple plans in one view.
In Plan, you can assign each task to a user and schedule it (add dates). Whenever a task has dates and assignee, you can see this same task assigned to the user in the Teams view.
With filters in Teams view you can filter out tasks from specific projects or simply look at all tasks at once.
The difference between Team and Plan timeline is how the tasks are organized - either by assignee on Teams view or Segments on Plans view.