A difficulty in recent discussions is that many folks are treating human acts as if they were an analog radio signal of effects and only effects which can be gradually attenuated down to nothing. They aren't. An act either categorically is deliberate remote material cooperation with grave evil, or it is not. It takes a certain minimum movement of the will to act at all.
If an act is deliberate remote material cooperation with grave evil at all, it can only be justified in the presence of a proportionate reason. And if the act is causally negligible with respect to the very outcomes which the person is analyzing under double effect in order to justify it, then a proportionate reason does not exist.
The contemplated act might be justifiable under some other understanding, of course. But it cannot be justified by appealing to double-effect with respect to outcomes upon which it has causally negligible effect.