It's a French expression. "Pendant" (noun) in French means "Objet d'art, de décoration symétrique d'un autre et formant la paire avec lui", and "en pendant" means "symétriquement" (hope you don't mind the French).
http://www.cnrtl.fr/definition/pendant

"Pendant" also exists in English with this meaning:

"pendant [...]
4A n artistic, literary, or musical composition intended to match or complement another.
‘the triptych's pendant will occupy the corresponding wall in the south transept’"
https://en.oxforddictionaries.com/definition/us/pendant

Here's another example of its use in a Spanish text:

"los dos cuadros que acabaron en Génova están en pendant con otras dos telas"
https://books.google.es/books?id=ViXPcFlhtfoC&pg=PA241&lpg=P...

But its use is pretty rare, even in art texts, and I haven't yet found one in English. So I think it needs translating. Referring to two artworks, you could say "as a matching pair". Referring to one of them in the relation to the other, I think you need to say "as one of a pair with".

"Landseer was commissioned to paint this picture as one of a pair with a painting of a Highland Lassie (RCIN 401516)"
https://www.royalcollection.org.uk/collection/401515/the-hig...

This example illustrates the use of "pendant" in English:

"The London Bridge pictures were often painted as one of a pair, with an image of the Tower of London or Westminster Bridge as a pendant."
https://en.wikipedia.org/wiki/Samuel_Scott_(painter)

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Note added at 57 mins (2016-09-16 16:33:06 GMT)
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Or maybe just "paired with"?