Calcium And/Vs. Magnesium

Ok, I seem to recall hearing that you should take magnesium and calcium seperately, since the receptors are the same, and taking calcium blocks the absorption of magnesium.
However, I always see magnesium and calcium supplements together, and I just read they should be taken together.

So which is it, and can anyone cite sources???

Calcium/Magnesium are a check/balance system. Hence why these two things are put together. Rule of thumb is usually 2/1 ratio for calcium/magnesium.