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I have a single inlet, dual outlet exhaust setup... The driver side outlet blows hot exhaust while the passenger side blows ice cold exhaust. What causes this?

Do you still have the heat riser on the exhaust manifold? If so, and it's stuck closed, the exhaust will simply go out through the intake. Exhaust will follow the path of least resistance.

The heat riser would not affect a single inlet muffler.
A transverse muffler mounted in line would have a straight through path on one side.

I honestly think the guy put a dual inlet, single outlet on instead

Yep. I missed that, I was thinking true duals. If it's only hot on one side, is it the side with the shortest run? If so, it's probably follow the path of least resistance. Hold a rag over the side that does exhaust and see if the cold side warms up.