As Cynder only appears in The Legend of Spyro and Skylanders, while Ember only appears in Spyro: A Hero's Tail and Spyro: Shadow Legacy, the two have not met in canon.
Both Cynder and Ember have had romantic feelings for their universes' versions of Spyro, although a difference is that Cynder's feelings are returned.
Although not massively popular, Cymber is possibly the most popular femslash ship in the Spyro fandom. It is less popular than their respective pairings with Spyro, Spynder and Spymber. Cymber is less involved in ship wars than the two pairings, and shipping the two is a more peaceful alternative to having Cynder and Ember fight over Spyro.
Cymber is mainly popular on DeviantArt and Tumblr and fanworks of it mostly exist in the form of fanart. On AO3, a tag for it exists, but the pairing is rare.
In terms of popularity timing, Cymber started gaining popularity considerably later than Spynder. While Spynder was already massively popular in 2007-2008, Cymber did not start to gain popularity until 2010 onwards. Most Cymber artworks are from 2016 or later.