(i)
Think of a ligand as a hand that grips the metal. An ambidentate ligand is one that has two different kinds of finger it could grip with, two different donor atoms, but it can only use one at a time. So depending on the situation it bonds either through one atom or the other, never both together. A clear example is the nitrite ion $\mathrm{NO_2^-}$. If it bonds through nitrogen it is called nitro, and if it bonds through oxygen it is called nitrito. The thiocyanate ion (S or N) behaves the same way.
(ii)
A double salt is made by mixing two simple salts that crystallise together as one neat solid. The catch is that this togetherness only lasts in the solid. The moment it dissolves in water it falls completely apart into every one of its separate ions, so it loses its special identity in solution. A common example is Mohr's salt, $\mathrm{FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O}$, which in water simply gives $\mathrm{Fe^{2+}}$, $\mathrm{NH_4^+}$ and $\mathrm{SO_4^{2-}}$ ions.