Q. I'm looking for information on the ritual use of a thod rgna, a tibet drum formed from crowns of two human skulls joined to form hourglass shape.

The Encyclopedia of Religion article on "Drums" makes cursory reference to the drum you describe (Sanskrit: kāpāla-ḍamaru):

General works on Buddhist drums are unavailable; and on drum symbolism, only Rinjing Dorje and Ter Ellingson's article "Explanation of the Secret Gcod Ḍa ma ruʾ: An Exploration of Musical Instrument Symbolism," Asian Music (1979): 63–91, gives Buddhist primary source material, a text on the symbolism of a Tibetan drum.

I suggest that you follow up on the above cited article in Asian Music in JSTOR.