When they marry each other, they are known as disaccharides.
a) Glucose lives with another glucose - they are known as
maltose (meaningful marriage)
b) Glucose marries the galactose - they are known as lactose (lame marriage)
c) Finally, glucose may marries the fructose - they are known as
sucrose. (marriage that sucks...)
If you have problem memorizing, just remember GUGUMA, GAGULA & FUGUSU.
GUGUMA = Glu + Glu --> Maltose
GAGULA = Galac. + Glu --> Lactose
FUGUSU = Fruc. + Glu --> Sucrose
Finally, don't forget that these processes release water -since they are married, so much 'water' would be spilled, heheheh ;-p