I bought headers for my 1997 honda civic DX and is has 2 down pipes to the start of the exaust so where would I put my defouler for my 2nd O2 sensor because there is not a hole for the 2nd O2 sensor.

 

you need to weld on a O2 bung

 

on one of the down pipes or where they meet?

 

After they meet. Make sure to put it at the top of the pipe, and not at the bottom of the pipe. If it's at the bottom and your engine spits out some unburnt fuel, oil residue, or condensation, it will damage the sensor and rust it.