And the practical reality of what happens to 2 so widely differing battery types in series means that it is not necessarily the best option. Unfortunately 'theoretical' options aren't always that simple in practice***... Then again, if you only use it occasionally, it may work well enough.

*** for example the small m/c battery will be of a lesser amp hour capability, and a different internal resistance, and it will flatten first, so you will then find you end up reverse charging it and damage it...