I have this exact make-up & if you order the vibram 700 half comp sole you will effectively get a double mid by default. As Linafelt pointed out, there is a slight recess in the leather mid so the outsole is flush. It’s a very clean & tidy way to do a half sole.
So to clarify, White’s uses a double mid-sole when boots are ordered with a half-sole rubber outsole. If you ordered a double midsole with a half comp, you’d get 3 layers of leather.