O2 sensor Bung?

[rs4pro3]

Ok I've got a set of E28 m30 downpipes to use on my m30 engine when I drop it into my e30. Now there is no Bung on it as the E28 has the bung on the cat and the Cat has been cut off these pipes.
I'm wanting to have a bung welded in on the pipes near the manifold like the 325i.
Now I know it wouldn't be best just to weld the bung to one of the two pipes, So I need to connect the two somehow and then have the bung welded in.
Any suggestions?
I may just purchase a Bung and take the pipes to a shop to see what they can do.

[brodee]

My headers just have it in one of the 2 sets of pipes, works fine. IF you do it just make sure it's in the set for the rear 3 cylinders.

[coupeowner]

Why on the rear headers only? Is that because it is closer to the heat source (pistons)? Or is there another issue? I had to move my 02 sensor about 8" forward on my headers to solve a Check Engine problem.

[rs4pro3]

Thanks for the tip, it will make life a lot easier if that will work. My best choice would be to add and X-pipe but to make it effective I need to calculate where it should be in the down pipe. Which I have no idea how to do.