Actually (speaking as a former military medic), you treat the one who can be most quickly stablized. But the general rule is to treat the personal with the most immediate life threat that is treatable. For example, if you have the choice between a woman in late stages of uncomplicated labor (i.e., the baby is coming within the next few minutes), a crashing asthmatic, and a gunshot wound to the head, I'd focus on the asthmatic. The GSW to the head is likely nonviable, the woman is in no dire distress (due to the lack of complications), and the asthma attack is most likely to respond to treatment. The head shot is likely to need to be tubed and bleeding controlled, but those can be done quickly but then the patient would be going to CT for further studies or to the OR. Do your ABC's on the GSW, then move on to the asthmatic, work on stabilizing that one and tell a nurse to notify you as the baby is delivering.
When in doubt, do the greatest good for the greatest number.