Ok,
Let me ask this question because I saw this discussed in another thread on here. When the water starts down the copper tubing it probably won't be 1/3 of the way through the copper tubing before it has reached the same temp as the wort, at this point it won't matter how much additional tubing there is, the water cannot hold any more heat. Two water sources will have more efficiency than one of double the length. So, a 25' 3/8 is great, and a 50' would be slightly better but 50' better than two 20'? I find that one hard to believe.
I ran this by a friend who is a HVAC and he agreed that the cooling efficiency of two 20' lengths would be better than a single 50' length.
Does this make sense or did I read your response incorrectly?