While convenient, epinephrine autoinjectors are generally limited to a single administration to either an adult or child.  The latest International Liaison Committee on Resuscitation (ILCOR) guidelines suggest a second dose of epinephrine may be needed within 5 minutes during a severe anaphylactoid reaction.  
 
Having two vials of epinephrine 1:1000 leaves you with several doses available for either an adult or a child.  However, you will need to draw up a specific dose, and administer it into the deltoid.  While less convenient, it is the same skill that MUST be used in the delivery of diphenhydramine.