Siamese twins

Siamese twins are either of a pair of identical twins born with their bodies joined at some point.

In 2372, Captain Kathryn Janeway described the USS Voyager and its duplicate as "Siamese twins linked at the chest", upon learning that the ship's antimatter had not been duplicated and was tethering the two vessels. 

In an alternate 2390, Harry Kim facetiously implied that lovers Chakotay and Tessa Omond were Siamese twins by describing them as "a couple joined at the hip."