Renoise 2.5 seems to lose multicore support randomly. You start it up and suddenly it has only one core available for processing. The other core seems to come back after a system reboot but then may be lost again after a restart of the program.
Both cores appear to be visible and operational in the Task Manager just fine.
The BIOS on my system is a fairly messy business, but from what I can tell, all that stuff is off.
I just started the program again, two cores are available now, but only one is selected. It is normally like that after a fresh reboot, but eventually one core goes.