This is a quote from "The wind-up bird and Tuesday's Women." How many fish tanks are there? How many trophies?
That would be one fish tank and two trophies, all of which are placed on top of something (その上). At least, that is how nearly every native speaker would read that sentence.
If it were the word 「ふたつ」 that was confusing you, it would not be used to count two totally unrelated items such as a fish tank and a trophy when there is one of each. It is not like you are seeing a green apple and a red apple, in which case many of us would use 「リンゴがふたつ（ある）」.