Hello Greg,
I tried it works. When we launch any 3D model(part or assembly) it displays in the model view. When you now add drawing to the existing session it gets added to Creo View. Since its a 2D drawing it will not launch and display within the model tab. You will find the drawing file added by moving from model to viewables tab or even the structure tab. Double click the drawing viewable and this will open in the same session. At this stage you can use the split scren option to view the model and drawing side by side.
Note that although you will be able to see the drawing viewable under models tab but, you wont be able to use the select checkbox or launch it from the models tab
Hope that helps.
Regards
~Syed