Put the summary of the bug you're having here

What I expected to happen was...:
the bottom piston would push the top one, the top one would rise. then on the
way back down. the top one would close and the bottom one would close

What actually happened was...:
same as above only on the way back down the bottom piston is still extended inside the top piston. top piston's grey base disappears

Steps to Reproduce:
1. make a double piston extender.
2. set the delay pretty quick between the two
3. open and close