Some investigations done, both fans are actually contolled by (bios)system. For whatever reasons, there’s only one fan interface and a switch between both fans controlled by writing either 0 or 1 to a config register. This is actually implemented in the kernel but has to be whitelisted for each notebook model and can’t be manually enabled by e.g. a module parameter:
[url]https://patchwork.kernel.org/patch/31041/[/url]
[url]https://lkml.org/lkml/2018/4/2/392[/url]
What makes things worse, it looks like Lenovo changed both fans to be independently contollable starting with the P50/51 which isn’t implemented yet if I understood this correctly.
So you can only go to the thinkpad-acpi mailing-list and ask for support.