My thought was that under certain conditions it's possible for an external cooler to cool the ATF below the temp of the engine coolant. So if you send it through the external cooler then through the original cooler it might actually heat the fluid up and be working against you....that's why i did it the same way that the original poster was planning. But i have to admit that i don't know typical temps for ATF or engine coolant...so i don't even know if that line of thought is correct...
