Not sure about myrrh but I am pretty sure frankincense is antimicrobial. Might kill all your yeast.
Yep... myrrh too. "So strong are the antimicrobial effects..." From TLC
another edit. Its possible they increase each others antimicrobial effects in a synergistic fashion when combined. PubMed
Another edit... I just realized this post is from 2011. I need to lay off the bottle tonight apparently...