I would be using a heathy UQ plus some sort of under cover vs a sock.
You can convert your poncho to a makeshift undercover by simply gathering the ends and mounting with shock cord. You can buy under covers from U2ZQ for about $35. I'm going to experiment with one made from polycryo this winder. I landscaped the back yard this summer and have my hammock poles set up, so I can experiment at will now.
I could see a sock having some advantage in windy conditions.
Double hammocks are good if you want keep a pad in place, but they aren't going to add much warmth. They do add some weight capacity. Note the chart that Warbonnet has on weights and single vs double hammocks: http://www.warbonnetoutdoors.com/travelers.php. Of course, you are going to double the weight of the actual hammock body, plus the extra expense.
You can certainly use a pad and an UQ at the same time. You can fold a space blanket and stuff that in between the hammock and UQ too--- no massive gain, but it has small cost and weight.
You can use a poncho or other light cloth for a top cover too. You want some breathing holes, so don't try to cover everything up tight. It's more of a windscreen. Using top covers will have the same issues with condensation as you have with tents. IMHO, of you get your tarp rigged nice and close, there isn't that much advantage until it gets really cold.
In my experimenting with under covers, I had few condensation issues. My guess is that is because warm air rises, there is plenty of ventilation on the top side, and no breathing to add moisture to air trapped below the hammock. The surface contact area for my body to transmit moisture to the bottom of the hammock is relatively small as well.
Search YouTube for "shug hammock". Shug lives in Minnesota and has hammock camped at -26F. He has a couple videos on cold weather hammock camping. He's quite the character too.