If a tactical marine with the Defensive Genius deed activates one of his brothers' chapter defensive squad mode abilities, can he then share it to the rest of his team using the Tactical mastery special ability? If so, what would be the complete sequence of required actions (free or otherwise)?

I think Defensive Genius ( which says "any defensive stance from another Chapter" ) and Tactical Expertise ("share the benefits of his Chapter’s Squad Mode abilities") do not stack together.

I think Defensive Genius ( which says "any defensive stance from another Chapter" ) and Tactical Expertise ("share the benefits of his Chapter’s Squad Mode abilities") do not stack together.

I agree, there's nothing in the wording of the rules to suggest they stack. Having both abilities enable the Tactical Marine (when leading the team) to 1) share the benefits of his own Chapter Squad Modes with every member of the Kill-Team, and to 2) call in Chapter Defensive Stances of Kill-Team members, but the effects will only benefit the Tactical and the member(s) of the Chapter whose ability was used.

I also agree. Defensive Genius gives some nice options but it isn't a shortcut to all killteam members getting access to all squad patterns.

Just for the record, the general solution is taking Forging the Bond (RoB, p. 226) - it is a must if you plan to share the benefits of Chapter modes.

Just for the record, the general solution is taking Forging the Bond (RoB, p. 226) - it is a must if you plan to share the benefits of Chapter modes.

I'm a fan of that talent. Sharing the patterns easily between the team is an ability so useful that it's worth paying for.