You may actually be running in dual channel now.
You have 2 sticks.
If they are in the right slots (check your motherboard manual) the two sticks you have will run in dual channel.
As far as going from 8 to 16.....IDEALLY you would by four 4GB sticks of the same set OR two 8GB sticks of the same set (that are also on the motherboards QVL list).
If you can't find sets that are on the list....any "set" is always better than mixing them.
...and then of course you can buy two 4GB sticks and match the specs to you other ones as best you can and try it and hope for the best....but this doesn't always work.