I never saw double bit axes to be honest i saw it have imagine it like weapon,not tooll. It is very dangerous tool becasause if it twist you have two blades on each side.
It woned me i saw videos on youtobeu when folks attach axe on handle they wedge it and than cut wood to be eaven with axe head. Why you do that, i would like to leave 1 inch 2.45 cm so it can pervent let's say axe that are too thin to attach like pickaxe (becaseu it is uncomfortable for hand if it is too tin).
Why just dont give some space for wood to expand more it expand it more holds.
Spliting mauls associate me to simple wedge that have handle in it. Axe for hewing should have blade edge reduction from thick to realy thin there is no pause.