Here you go:
Click to show science
Let’s imagine a triangle of the three things that matter here: The target, the proximity explosive which is triggered by the target, and the secondary explosive which is triggered by the proximity explosive.
The sides represent the direct travel path of the explosions. This image should make it somewhat clear:
First image before the proximity explosive is triggered. Second one the proximity explosive was triggered and it’s blast reaches the secondary explosive. Third picture the target dies - by the proximity explosive.
While playing with the camera tool mod on PC I noticed, when in slow motion, that the blast is actually not instant but takes time to grow from the explosive’s center.
If we assume that the blasts of all explosives have the same travel time, it becomes clear why we cannot kill targets with the secondary explosives. The sides a and b would need to be shorter than c. But this is impossible. The two shorter sides of a triangle combined can never be shorter than the longest side. They can only be equal if the two angles at the longest side are 0°.
Practical result: not possible.
Theoretical result: If you could place them on an exact line within a developer map editor, it is possible to make the blasts reach the target at the same time, resulting in a “maybe possible”. But maybe there is a delay between “Secondary explosive triggered” and “Secondary explosive blowing up”. In which case it is impossible in any way.
EDIT: Oh wait it MIGHT be possible if the proximity triggering reach is bigger than the explosion reach. But I never saw that being the case.