@tecnicasopr,
Incorrect ... if MultiMonitor Mode is not enabled then the IGPU will be disabled if a dGPU is present and set as primary.
The BIOS does not know about headless mode only if the IGPU should be enabled or not. The Headless configuration is performed by MacOS via WEG (or Clover) not by the BIOS.
I've always found the term Multi-Monitor Mode to be a bad description of what the function actually does.
It should really be called "Mixed GPU" mode (IE: IGPU + dGPU)
If you intend to use the IGPU as headless and the BIOS has a Multi-Monitor mode setting then it should be enabled.
Cheers
Jay