If I take Vitamin K2, should I also supplement with Vitamin D3?

In many cases, taking K2 with D3 would be beneficial .. however it does depend on your diet & lifestyle.

If you work outdoors, or spend a good portion of the day outside with your skin exposed to the sun, you may not have a need to supplement vitamin D3.

And if you eat foods rich in Vitamin K2 like natto, chicken thigh, egg yolk, some cheeses (Gouda, brie, edam) & grass fed butter, you may not have a need to supplement vitamin K2.

So it's best to take into account both your diet & lifestyle when considering whether you should also supplement vitamin D3 with your vitamin K2 supplement.