Generally light weight 2 person tents require the use of 2 20 inch pads to avoid overlap when used by 2 people. Some pads are tapered also which helps with this and may allow the use of a wider pad. Also it is generally more comfortable to use a 3 person tent for two people and that will allow the use of wider pads.  

Less lightweight 2P tents generally have a rectangular floor designed to allow 2 25 inch rectangular pads.

I doubt custom pads are available for general purchase. It is possible to cut down and reseal some inflatable pads which some people do mostly to save weight.