(i)
An ambidentate ligand is one that carries two different atoms able to donate a lone pair, yet it can use only one of them to bond to the metal at any given time. It is like a key with two tips where only one tip fits the lock at once. A clear example is the nitrite ion $\mathrm{NO_2^-}$. If it bonds through its nitrogen it is called nitro, and if it bonds through an oxygen it is called nitrito. The thiocyanate ion $\mathrm{SCN^-}$, which can attach through either sulphur or nitrogen, behaves the same way.
(ii)
A double salt is a salt that is perfectly stable as a solid crystal, but the moment it is dissolved in water it splits up completely into all of its separate ions and loses its own identity in solution. In other words it exists only in the solid form. A standard 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 just as the individual salts would.