Select the 2D parent part.

Select Create → Separation Attachment lines...

Bring up the dialog defining the necessary variables by clicking the Define sep/attach variables... button.

Define either Density and Momentum or Velocity, as well as the Ratio of Specific Heats.

The variables can be set by either typing them into the fields or be selecting them from the list above and clicking the Set button.

Click OK to finish the variable setup.

Click Create with selected parts.

This will create two parts - one each for the separation and attachment lines. You can modify the visual of these parts separately, but when you change any creation attribute, both parts will be modified.