Adding partition to disk creates unwanted third partition
Use the following steps.


Open the Disk Utility application. Highlight the appropriate drive.
Below is an example.


Click the the icon labeled Partition. The following popup pane will
appear.


Determine the size you want for the new partition. Subtract this
value from the size of the current partition. In this example, I
need a 30 GB partition, so the difference will be 69.656 GB. Replace
the current "Size:" value with this difference, then press the
return key. Below is the result.


Highlight the new partition by clicking on the pie piece. 


First, select the "Format:", then enter a name in the "Partition:"
field. Finally, click on the "Apply" button.


When finished, click on the "Done" button.


The final result is shown below.