  In the last years, photometric and spectroscopic evidence has demonstrated
that many, maybe all the Globular Clusters (GC) host multiple stellar
populations. High-resolution spectroscopy has established that, while most GCs
are mono-metallic with no significant abundance spread in $s$-elements, in all
the globulars studied to date the presence of different stellar generation is
inferred by the Na-O and the C-N anticorrelations. In this context, NGC 1851
and NGC 6656 are among the most intriguing clusters. Contrary to the majority
of GCs, they host two groups of stars with different s-elements abundance that
are clearly associated to the two distinct sub-giant and red-giant branches
detected in their color-magnitude diagrams (CMD). In the case of NGC 6656
s-rich stars are also enriched in iron and calcium. Each $s$-element group
exhibits its own Na-O and C-N anticorrelations thus indicating the presence of
sub-populations and suggesting that the parent clusters have experienced a very
complex star-formation history. In this paper we summarize the properties of
multiple populations in NGC 1851 and NGC 6656.
