Is 1 propeller always more efficient than 2? Based on other answers I've read here (1) (2) (3), a single large blade propeller is the most efficient type of thrust you can get. 

There are 2 reasons for this (as I understand it). Firstly, because each blade on the propeller causes wing-tip vortices when low pressure air meets high pressure air. 

Secondly, each blade on the propeller adds surface area to the aircraft increasing parasitic drag. 

So my question is what if both propellers are surrounded by a duct? Assuming it's a perfect fit, the duct will neutralize wing-tip vortices, so now all that's left is parasitic drag.

Now couldn't you just use 2 propellers that equal the surface area of a single large propeller and get the same efficiency with no added parasitic drag?

On a similar note, could you use just 1 small propeller surrounded by a duct and reach similar efficiency (if not better) than a large propeller surrounded by the same duct? 

I say "if not better" because the smaller propeller would have less surface area, thus less parasitic drag.
Ducts are only used for highly loaded propellers of a very small radius, namely the fan stage of turbofan engines. For lower disk loadings a shroud or duct would create more friction drag than it saves in efficiency.

If the tip Mach number of the propeller is low enough (say below Mach 0.9), then the single propeller of maximum diameter will be the most efficient way to create thrust. This condition is met when both flight speed and thrust per propeller disk area (disk loading) are low.

As the aircraft gets faster, the tip Mach number can best be limited by reducing the propeller diameter. Now more blades will be needed in order to create the desired thrust, and interference between them will reduce efficiency. Also, the higher disk loading will result in more swirl losses, and now it becomes attractive to run a second, counter-rotating propeller behind the first. The two can be either close together or - even better - on opposite ends of the aircraft.