by parallel I assume you mean one fan blows into the other. This is not optimal unless the two fans spin in opposite directions. Think of it like this... the air is already spinning around from the first fan so the second fan is going to spin it more in the same direction, reducing the effectiveness of the 2nd fan. If the second fan spun in the opposite direction, then it would be spinning the wrong way-- increasing the efficiency of the 2nd fan. But unless you have such a setup, it is best to put the rad in the middle... the rad acts as a straightener so the 2nd fan gets straight air, not a vortex.
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