On the far right side of the screen, under Properties, click on ADD AREA

Select from the list in the pop up and click ADD TO START or ADD TO END

The additional area will appear in your plan view as well as under Properties

To amend Area 2, click on the "2" under Properties. It should turn blue and say AREA 2

To amend the join type, click on the join icon between 1 & 2 under Properties

From the list below, select your join type and add a join offset if required

When selecting various options you may receive an additional prompt like below. Confirm or reject as per your design preference

You can now amend the products or dimensions of your structure either by selecting on 1 or 2 under Properties and making amendments or you can click on the gold coloured dimensions in the plan view drawing and amend as required