Hello Ported! 

I'm quite certain it's a system constant from compilation at the factory
SY STERVK PROKON EIN system constant condition: stereo exhaust system upstream of cat

Simple solution might be to make sure in code that lamsoni_w also get written lamsoni2_w memory position. Though I have not checked the FR yet if that is the correct variable and if it is as easy as this.
That way you would only need 1 pre cat lamda in downpipe. Perhaps that is what c2 used to do to solve the situation for all complaining eiptuining vr6 24v customers a few years back. But you need to startup IDA to fix this.

preparing for a tubular manifold with no possibility for stereo lambda pre turbo?

Us single turbo 2.7t guys have been messing with this for a while, so far a solution has not been found other then to run o2 pre turbo.  I will investigate the lamsoni_1 idea.

It can be solved at the point where the second O2 voltage is read.
Just NOP it out and copy over the first O2 voltage instead, job done.