Stack Groups  
Multiple Area series can be stacked, one on top of another.
Additionally, stacking can be made in groups of series.

You can assign a group number to each series you want to stack together:

Series1.MultiArea := maStacked;
Series2.MultiArea := maStacked;
Series3.MultiArea := maStacked;
Series4.MultiArea := maStacked;
// Now create two groups: 
Series1.StackGroup := 0;
Series2.StackGroup := 0;
Series3.StackGroup := 1;
Series4.StackGroup := 1;