Hi Cassie - I joined a Vitamin D Deficiency and Treatment Facebook Closed group (18000+ members) that use research based articles as their information.
Magnesium is needed as this is depleted when the body uses the Vitamin D. Vitamin K2 is important to keep the Vitamin D in the bones and teeth and to prevent high Calcium levels in the blood, this is what people would call 'Vitamin D toxicity'. Other supplements also discussed eg Boron.
The doses are an individual thing depending on your Vitamin D dose and your own tolerance - Magnesium Glycinate 400mg (too much can give you loose stools). Vitamin K2 comes in two forms Mk7 or Mk4 and their daily doses are differ. The K2 is more important if you are on the high doses of Vitamin D.
Currently I take Vitamin D3 6-8000 iu, Magnesium Glycinate 200-400 mg and K2 Mk4 5mg a day.
Hope this helps a little -
PS If you are interested in reading a bit more google 'Dr Coimbra' who is a doctor using high dose Vitamin D therapy to treat autoimmune diseases eg Multiple sclerosis, with success.