In order to merge two designs together, it is recommended, but not obligated, to first duplicate one of the designs. The reason being that there will always be an original copy of both plans as a "single" room design in case it is ever required. Here is a link that shows how to duplicate a design.
How to Duplicate a Design in 2020 Fusion
1. Open the two designs that require to be merged.
2. Decide which room should be copied in order to be pasted into the other room.
3. Once decided, ensure to be in Plan view and press CTRL + A on the keyboard. This will select all the items in the design and allow it to appear like or similar to this.
4. With all items selected, press CTRL + C to copy all the items.
5. Minimize the plan that was just copied.
6. Ensure the plan that the items will be copied to is active and in Plan view.
7. Paste the copied items into the plan by pressing CTRL + V on the keyboard.
8. The pasted items will then appear and will still be selected.
When the items are still selected after they have been pasted, it is easier to rotate them or place them as a group to properly align them with where they need to be.