Here is my initial setup, two pistons pushing one force relay bound to the chisel factory block

This is the result that I got after activating both of the pistons with the levers.

This is the result I intended.

Varying the timing of the levers does not change anything.

It appears that when a force relay is pushed by a block that is being pushed by a piston, the push does not register. If it is directly pushed twice by a piston, both pushes register

Welcome to the incredible world of force relays, where their very existence is a variable and the fact that they even work is a miracle.

On other words, can't fix, won't fix.