In some cases, you may get away with one cooler mounted out of the air stream. Those are rare cases.
Let me relay a story of a fella. Granted he had a heavier vehicle, but that doesn't mean you shouldn't take note of his experience. It's a good lesson.
This customer came to me months after we had rebuilt his transmission and was complaining that something was wrong with the transmission. He felt it was getting hot. I drove it and yes, on a long uphill pull it would get hotter than I like to see. It also had a long temp recovery time. What I mean is, after it got hot, it would take a very long time for the temp to come back down. Even though you might be going downhill at that point. Of course, the first thing I looked at was his cooler setup. He had the stock radiator cooler, two auxilliaries parallel to the ground, behind the front bumper and one beside the trans under the drivers location. It was the one beside the trans that had a fan. It was circulating heated air trapped in the undercarriage. He felt he had plenty of cooling and that something definitely was wrong with the trans. My first clue was the long temp recovery time. I told him to clean up his mess and run ONE auxilliary behind the grill along with the in radiator cooler and toss the rest. He resisted my advise for months. He had us install a custom deep pan, because he felt that would help cool it. Even though I advised him that would do nothing for his problem. Still prodding him to change his cooler arrangement. Well, one day he drove in. Happy as a lark. His trans temps were acting normally and he had a box of donuts for us all. I'll let you all guess what he did to change his situation.
By the way, that was not the only time that has happened to me.
