So, say Alice her money was transferred first, and bob's money second. Then, after you've spend N coins, anyone looking for bob's coins sees you no longer have

So, say Alice her money was transferred first, and bob's money second. Then, after you've spend N coins, anyone looking for bob's coins sees you no longer have them. Anyone looking for Alice's coins sees they are still with you. This is because it is FIFO, so the outgoing transaction is only spending from Bob's coins.