Technically there is no such thing as a bitcoin. Just unspent transaction outputs. Those get spent as an input of a transaction and then are gone forever. There is no concept of the output of a transaction being the same "bitcoin" as what comes from the input of the transaction. This means if you had 2 inputs and 2 outputs of the same amount there is no way to trace which input became which output. At best you can find which outputs potentially came from an input.