This is like Melded Tandem/As couples, except that some people may be in couples and the others in tandem.
The person whose spot you are paired with might be T-boned to you. This makes it much harder to identify the spot you should consider yourself paired with. The most common case is a 2x4 setup, in which case everyone is paired with the other spot in their quadrant.
\syhlines{b4e,g3s,b3s,g2w}
\sxvlines{g1e,b2n,g4s,b1e}
before Melded Siamese Split Swap
after
There are a few other setups in which it is clear from the geometry how people should be paired.
\syhdiamonds{g3s,b3e,b2s,g2w}
\sxetees{b2n,g1s,g4n,b3s}
before Melded Siamese Flip the Diamond
after
\sxwtees{b2s,g1n,b3s,g4n}
\sxetees{b2w,g1w,b3n,g4s}
before Melded Siamese Switch to a Diamond
after
\sxhlines{g1n,b2e,b1n,g2e}
\syhlines{b4s,g4s,b1w,g1w}
before Melded Siamese 1/4 Left
after
Overlapped Siamese is an obsolete name for this concept.