When looking at a medication order, the dose ordered may require two (or more) vials of medication to make up the correct amount to administer to the patient.
There should be no guess-work involved. The correct concentration at which to make up the medication is given in the Australian Injectable Drugs Handbook. This invaluable resource should be consulted for every injection that requires mixing before administration.
The handbook also contains information as to the compatibility of the medication with other medications and whether it should be mixed with water or can be mixed with saline or other solutions.