Def allies contribute 25% of their standing military strength towards your defence. The sum of both your def allies' contribution is capped at double of what your own standing military strength is.
assuming both def allies contribution did not double own military strength and assuming non-dictator, the calculation for defence is it [own defence points+(allies defence points/4)]*1.1*(weapon tech)*(def bonus) OR [(own defence points)*1.1*(weapon tech)*(def bonus)]+(allies defence points/4)? i always take the former just in case.. it will be good if it is the latter:)