This combine is done using your fletching kit. I would assume the borrowed medicine bag is used only to combine the blood in once you've made all the other combines as well as for the other captured animals.
That would be correct, I completed all my captures and made the blood samples in the fletching kit while the medecine bag was in the bank. It had no effect on spawning the monsters either. I only used the bag after completing all the blood samples, and did it right at the bank in PoK where i picked it up and took it straight to Corun in Surefall Glade.