I second Mac - the second dose was impractical if you had to self-administer. EpiPens and Allerjects are difficult enough when you are mentally and physically compromised. Plus, even if I was alert enough to give a second dose, I'd be shaking too much from the first dose to do anything that required coordination. (Every time I've had Epi I got a severe case of shakes/shivers.) Thus, we never chose it when given the option.
Most first aiders I taught were pretty leery of the second dose as well. Seeing/using a syringe is scary for people.