How do I send data from one command block to another

Testforblock doesn't check what block is at a certain coordinate, it confirms whether or not the specified block is in the specified position

testforblock <position: x y z > <tileName: Block> [dataValue: int]

testforblock will check for a block and the command execution will succeed if it finds the correct block. otherwise it will fail so you would have to make the second command executing conditional on the first one succeeding.

A method to do this is to make replaceitem a conditional command block. which would only make it execute after testforblock found whatever it is you were checking for.