The area of the house is not the major factor.. it is walls and floor/ceiling.. and construction type.
You will not get wifi easily through brick, concrete or metal. Even through standard hollow wall you need to expect 20% signal loss and worse through floor/ceiling it is double storey.
The main answer to getting better wireless over a whole house is to wire it with ethernet. Ethernet connect a few Airport Express as WAP.. wireless access points.
Do not even attempt to use extend wireless with 10 express.. it is far too many and they will interfere with each other.
You also need to understand Apple routers cannot work as a mesh.. each express must get good signal from the Time Capsule and much less signal from all the adjacent Express.. if you have even one express too close to another.. then wifi extender will attempt to swap to the highest signal which will then break if it is another extender.
Running ethernet and setting each Express up as independent WAP fixes this .. you can then run different channels from different WAP and not have slow double hop or triple hop wireless.
Using two modems is not possible.. but you can simply setup the second modem so if the first modem has issues they can be swapped over.. The backup modem be turned on and running in the network.