wouldn't the 2 piece header leak at the sliding joint and affect the o2 readings when tuning?

I had try-why rep and welded them shut because I could see leaks on it

now I got a spfab toda replica header that's also a 2 piece design

 

If the tubes are a snug fit you should get next to zero leakage. More exhaust would escape than would be drawn in on average due to pressure within the tube. A larger surface leak would be much more likely to affect O2 readings.

Bending a single tube that matches becomes much harder the more times you bend the pipe. A shorter pipe with a single bend can be correctly more readily than the same tube with 2 or more bends.

Ideally, the tubes ends would be chamfered to seal easier and with the aid of the springs, prevent most leaks.

Did you notice a difference in exhaust readings welded and un-welded? If so, please explain what sensor you were using.

 

the joints also become coated with carbon over time and that helps seal them, you might get a very slight leak at first but it will go away

 

not with this header I just got it not on the car yet

with the ebay try-why I had to weld everything up it was flimsy as ****