When two lenses having equal but opposite power are next to each other, their combined power is zero or plano. They neutralize each other. The curvature of the minus lens where ray #2 enters it is exactly the same as the curvature of the plus lens where ray #2 exited it.
 
 

When the minus lens is moved away from the plus lens, the curvature of the minus lens is much less where ray #2 enters it than the curvature of the plus lens where ray #2 exited it, and so there is less refraction occurring in the minus lens than there was in the plus lens. So, the minus lens has effectively lost minus power.
 
 

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