Putting 2 observers together facing each other creates an asynchronous redstone clock. Joining the redstone of these 2 signals synchronizes the signals to create 1 uniform pulsating signal. This is true for all blocks except for affected blocks.

AFFECTED BLOCKS: Scaffolding, Hoppers, Ice blocks (variants work normal), all stair variants, all slab variants, all glass variants, observers, and downward facing pistons

On these blocks, the signal remains asynchronous if joined together using a signal that goes over an affected block.

Visual reference for how to link the signals (the lines being the redstone dust):

(air block air block air block air )________________(air block air block air block air block )
links to one observer output ___| affected block |___ links to the other observer output

Steps to Reproduce:
1. Place 2 observers facing each other.
2. Place one of the affected blocks.
3. Link the 2 redstone signals (from the back of each observer) by placing redstone dust on the top of the affected block and over it. (there should be redstone on the side of the block indicating that the redstone on the block and on the floor are joined together)

NOTE: Joining the redstone signals by any means other than over an affected block causes the redstone to behave like normal.

By following these steps you will notice that the redstone on the floor is pulsating at a different speed than the redstone on the top of the block, despite them being connected.