Hello elle_h!
Welcome to our Mattress Forum! 
In very general terms, all would depend on the various durability factors which include the type of latex, the softness of the layers, and any other materials in the mattress but if all else was equal and based solely on construction … then a two-sided mattress will be more durable than the same layers in a single-sided mattress yes … even with latex. It won’t be “twice as much” because all the layers compress in use whether they are on the top or bottom of the mattress but it would make a significant difference IMO. There are always tradeoffs involved though because a two-sided mattress has some design tradeoffs because it can’t use thicker softer layers in the comfort layers to the same degree because when they are on the bottom it could affect alignment (it would have the softer layer on one side for comfort and also on the other side which would be part of the support and this can result in higher amounts of softer latex in the mattress).
There is more about one sided vs two-sided mattresses in post #3 .
You are not mentioning the type of foam used in the quilting and comfort layer of mattress A. Without knowing complete mattress details including the density of each of the foam layers it is impossible to say which one would last longer. Even with a complete set of specifications it would be difficult to assess with certainty the durability of mattresses using similar materials because of other qualitative factors such as the way the mattress is manufactured, if it has replaceable layers (Some mattresses have individual layers and a zip cover), glued layers or not, seams, tape edge, and other more “arcane” factors such as the shape of the foam cells, the strength and elasticity of the crosslinks, the resilience of a foam, and the compression modulus, etc… )
The latex components are high-quality materials that are very unlikely to reduce the durability or useful life of a mattress, which you likely learned when you read the Mattress Durability Guidelines but durability is also affected by the firmness of the layers with firmer foam being more durable than softer foam.
A lower gauge means the mattress has a thicker coil, and therefore a firmer spring, but an innerspring isn’t normally the weak link in a mattress and the gauge of steel is only one of many factors that determines how a particular innerspring will feel and perform inside a specific mattress design. There is more about the different types of innersprings in this article about innersprings and in post #10 here but I would pay much more attention to the quality of the materials above the innerspring which is normally the weakest link in a mattress.
The “weakest link” in a mattress in terms of durability is normally in the upper layers (the top 3" - 6" of the mattress) that are compressed more deeply when under the weight of the sleeper and not generally in the deeper support layers so the firmer bottom layers of a mattress will have more effect on feel and performance than they will on durability for most people.
I hope this helped clarify some things re durability. Please keep us posted on your mattress purchasing journey!
Phoenix