But what might be the simplest circuit you could envision for this purpose, it might still be within my abilities.

Given your constraints, and the fact that it is a noisy AC signal, the only thing that occurs to me is to use a high frequency signal from the trailer light to operate a switch at the bike light. If it is present, switch off; otherwise behave NC.

This is "simple" but not in the sense you mean.

 

So I thought about it. And here would be my idea. Mosfet P to switch the first lamp. Would this work, and if so, what would I need to take into account (resistors and capacitor wise).
Thanks in advance.

 

Without looking too deeply, the top right diode of the bridge is connected directly across the generator output so it will short out the negative half cycle.

 

My first thought is to use relays - i.e. When current goes through one loop, open the other, when the current through the first loop stops (opens connector) the other loop closes

 

 

The relay coil is in series the lamp so they must share the voltage and there is only 6V available and the lamp needs that. You would need a very special relay to do opertae but leave most of the voltage for the lamp.

 

>there is only 6V available

I thought that the globe was 6V and the generator produced 1V-10V?

 

So I thought about it. And here would be my idea. Mosfet P to switch the first lamp. Would this work, and if so, what would I need to take into account (resistors and capacitor wise).
Thanks in advance.
View attachment 235580

You indicate a switch. why not replace with 3 position

 

How do you turn on the First Lamp?