If you are filling larger areas, you do not use mortar, but should use grout. The grout is like concrete, but with smaller maximum size aggregate (sand/aggregate, cement, water and no lime). It is very fluid, with a slump of 8 to 11", compared with concrete that is 3 to 4". The high moisture content and the small aggregate allow it to fill the voids. The excess water is absorbed by the masony units, but the grout should be placed in lifts of several feet, allowed to consolidate and the be topped off the finish.
A basement wall of two layers is very unusual.