In my example there are 3 identical rigs, maden of 6 bones.
FK A and FK B bones have a custom shape, as they are superimposed to DEF A and DEF B.
FK B is child connected of FK A.
Child A is child of FK A.
Child B is child of FK B.
DEF A has a stretch to bone constraint pointing at child A.
DEF B has a copy loc bone constraint copying loc of child A and a stretch to constraint ponting at Child B.
In the second rig FK A and FK B have been posed as a standard FK rig.
In the third rig a similar pose, tweaked using child A.
