It's jargon used among Japanese アイドル fans, especially ジャニーズ fans.
シンメ is from シンメトリー ("symmetry") and initially referred to performers dancing symmetrically in the mathematical sense, like a mirror-image.
Then it gained a broader sense among idol fans, and it now mainly refers to two idols within a group who often perform together and are considered as a "pair". For example, the person in the video mentions やまちね (山田涼介 and 知念侑李), who can be considered as a シンメ in Hey! Say! JUMP. Most people in Japan know nothing about やまちね and シンメ, so I think the person in the video said it to show his knowledge about J組.