There are 2 different way to combine to shapes into a single shape in Sure Cuts A Lot & eCAL. One is to use the Weld option in the Style tab, and the other is to use the Union from the Path menu.
The main difference is the Union permanently creates a new shape, while the Weld temporarily creates the new shape when you do a Preview or are actually going to cut your project.
Below is an example of using the Weld option on 2 circles. Select the shapes you want to weld together, and then check the Weld checkbox from the Style tab.
Now if we do a Preview, you should see what is going to get cut in Red, and you can see the parts that are ignored in gray.
Because the Weld is not altering the shapes, we can easily rearrange the overlapping shapes if we want.