I watched this anime 10-15 years ago.
In the episode I watched the enemy was a woman and man that merged their bodies to create a singular body which was a super powerful version of themselves in order to fight their opponent. The woman was saying something about their power of love and how their power comes from the strength of the love between them. I think they may have used a gem to merge their bodies.
Also I believe this anime might have revolved around these types of merges by the hero's as well who also may have used a gem but I'm not sure. I only remember this specific scene.