M.A.D for a qualified line clearance arborist would be at least 2.5 feet if not 3 feet (add 6" more to those distances for elevations greater than 5000 feet) for the primary line(s) and 6" for the secondary line. I have seen some spans that you likely could go between the primary and secondary but I don't think that is the case with most of the line that I see. I have seen my crews do it on the rare occasion where they can maintain more than minimum clearance but if not they use their insulated tools from above/below or get the throw ball and gear out and climb it. It's not worth taking chances with.
There is a new rule about passing between wires that says something like "MAD is to include operator's potential reach" which makes passing between wires out of the question. Which is why all the utilities are going to elevators.
Company policy may play a role in this but if your a utility arborist directly working on line clearance you can. Secondary wires are "no touch" and as long as you can keep your approach distance, including your reach as stated a few posts before this, it is acceptable. Although when I was doing line clearance we were crouching under everything and minimum approach got broken on a daily basis. Not proud but it happens. Keep your insulation up to code with inspections, know your circuits and voltages and respect the lines