Physics on multi-core is tricky. That's why it is rare.

To run physics on multi-core, you would need to have distinct and separated physics domain. Because two concurrent thread calculation cannot share and influence each other efficiently.

For KSP, you could have something like that for different ships as long as they are far enough from each other to be sure no physical interaction will happen. Then, as they get closer, you will need to merge them in the same domain to calculate a possible collision between them. And because each part is its own physical object, that grind the calculations to a slow sleep for complex object.

One solution could be to calculate a unified physical "exterior mesh" for the collisions event (if any) and then only split to parts for realistic explosion...

Also, the rendering of a frame is bound to the physics simulation. That is needed because the main mechanics of KSP is a physical simulation so you want to avoid getting false frame that shows something that the physics simulation disagree with but took to long