Help please!

Two associative functions on the the unit square:

[0,1]^2 -->[0,1]

x,y --> T(x,y) and S(x,y)

What is the ordinal sum of these two functions on [0,1]^2 ? I know it is not T(x,y)+S(x,y)

Does ordinal some have something to do with ordinal numbers?