When you are using two different colour strands in double knitting you have to move both strands to the front or back depending on whether you're knitting or purling.
You're basically doing St st but with both colours and the RS will always be facing you, i.e. with colour A you will always be knitting on the RS and with colour B you will be purling as it will be for the WS.
The colours are interchangeable and this is known as the 'jacquard technique'.
I found it a fairly easy technique to pick up though you will probably find that your first attempts will be loose.
The best explanation for this, I've found, is in the 'Big Book of Knitting' by Katharina Buss and is under the category 'Reversible Patterns'.