1. Equip an Exotic Emote
2. Go to inventory and change the emote to an emote you wish to merge the exotic one with
3. Once you equip the second emote, instantly close the menu and press left on d-pad, you should still use the first exotic emote you equipped
4. While in the animation of using the exotic emote, either wave, dance or sit down, then press left on the d-pad again. This should use the second emote you equipped, but still show the exotic emote's holograms or particles.
This doesn't always work.
Worthless i know, but there are some funny combinations to show off to your fire team for those that still play, such as the exotic emote six shooter merged with shadow dance.