Calcium and magnesium: better taken at different times or together?

Magnesium plays an important role in bone health, as noted in an article by the National Institutes of Health. For years I have been taking both calcium and magnesium supplements for bone density. However, I was surprised when my pharmacists remarked that magnesium and calcium should be taken separately so that they don't bind to each other, preventing absorption. 

Reading a little online, there was some confirmation of the value of taking magnesium separately. One benefit of taking magnesium right before bedtime is that it can have a calming effect.  

Magnesium can be absorbed through the skin and is available in sprays, oils, and bath salts. A nice alternative to pills! I have not yet found a good review article about transdermal forms of magnesium but will keep looking.