Why would I want to use both power supply inputs on the Boss TU-2???

I believe one is actually a power output that you can use to power your other effects.

 

One is the input for the power supply. The other is an input for a daisy chain that powers more pedals. Look it up, it's all on the web. The TU-2 and daisy chain is how I powered my pedals for years.

 

Yep. One is an input. One is an output to plug a daisychain into to power other pedals. Careful though. If you're using the boss adapter to do this, it only supplies 200mA and the tuner itself take up something like 50mA. If you get a few digital delays in there or something, you'll start running into trouble. You could, of course, just use another adapter.

 

This is a gag thread, right? It's in the manual. There isn't an "in" and an "out" you can use either one to plug into and the other to daisy chain.