Re reason 2: if you were only wearing one speedo the other speedo would remain dry in your bag and so not suffer chlorine degradation.
Since chlorine is a gas dissolved in water it will travel through the outer speedo (except for that part which attacks the outer speedo) and then attack the inner speedo. The outer speedo will not significantly protect the inner speedo from chlorine.
If speedo fabric were close knit enough to act as a semi permeable membrane it might be possible to allow water to pass through while excluding chlorine, but your speedos would fill and expand with chlorine free water so you would gradually appear to be wearing a colostomy bag.
Re reason 1: The original reason for wearing 2 speedos was to achieve drag while swimming. At one time, pre nylon speedos swimmers wore narrow jockstraps under their trunks:
http://www.ebay.co.uk/itm/JOCKSTRAP-...em2 c6e4f4bb8
for support, swim jockstraps are rare these days as nylon and lycra speedos did not require them, but in a contact sport like water polo where your speedos may be ripped down suddenly, an extra pair underneath may help.