My thoughts are as follows: if you have the ability to allow two or more threads to control your game (update/render), go for it. But let the OS decide which thread runs on which core. However, if you can't control which thread goes on which core, then a problem arises. If the OS asigns both threads to the same core, they won't run simultaneously. Say this is the OS's thread queue, in which case the OS assigns each thread to a differant core:
CORE 1:
Process 1
Process 2
Process 3
Process 4
Process 5 (Your game's UPDATE thread)
Process 6
Process 7
Process 8
CORE 2:
Process 1
Process 2
Process 3
Process 4
Process 5
Process 6
Process 7 (Your game's RENDER thread)
Process 8
This would be great, however, if the OS assigns your threads to the same core, it would be like your game is running on a single core system, each thread would have to wait for the process ahead of it to finish, and would have to wait for your other thread to finish. Here's how the OS's thread queue might look in that situation:
Process 1
Process 2 (Your game's RENDER thread)
Process 3
Process 4
Process 5
Process 6
Process 7 (Your game's UPDATE thread)
Process 8
My point here is, if you can't control which thread goes on which core, just use a single-thread game, or it may run slower if your threads are running on the same core (or even 3 are on one core, and 1 on the other, etc).
Take the Xbox 360, they have "3 symmetrical cores running at 3.2 GHz each" and "2 hardware threads per core; 6 hardware threads total" (source:
http://hardware.teamxbox.com/articles/xbox/1144/The-Xbox-360-System-Specifications/p1). This means they can run 6 threads sumultaneously, however, it's using custom hardware, and the hardware is dedicated specifically to the game. So the 360 obviously makes use of these 6 threads running at the same time. I bet the 360 has the ability to choose which thread goes on which core, or the 360 automaticly assigns each new thread to a core which does not already have a thread running, meaning it cannot allocate more than 6 threads. Note that this is only my guess, I have NOT confirmed this.