First you have to find molar mass for water in general.
When you have 2 molecules ---> 2
There is no such thing as a molar mass of 2H₂O.
The molar mass is by definition the mass of one mole.
The mass of 1 mol of H₂O is (2×1.008 + 1×15.999) g = 18.015 g.
So the molar mass of water is 18.015 g/mol.
Of course, the mass of 2 mol of water is
But that's not the molar mass of 2H₂O.