Tobramycin & Timentin running through IV line query?

I don't have my med book close by, but will give you an educated guess. Some meds will actually "coat" the IV tubing so if thse meds are not compatible and you have run the Timentin through and then run the tobramycin you may have crystillization even though you may have maintenence fluids going through the line.
Again I repeat That I did not look this up but have experienced similar information regarding other things in the past.
Example being If we had maintenence fluid infusing through a UVC the dextrose would coat the line so even though we might be able to draw some labs through this line a glucose would be unreliable even when drawing out waste.
Don't know if this helps or not but it is one possible explanation.

Mixing Timentin with Tobramycin is contraindicated, as you stated. Timentin can substantially inactivate the Tobramycin.

The Timentin should be infused AFTER the Tobramycin. It can be infused with a Y tubing/piggyback and all other med solutions must be temporarily discontinued during the 30 min. infusion.