Before importing your stereoscopic content in the editor, you need to add "_TB" (top-bottom) or "_BT" (bottom-top) or "_LR" (left-right) or "_RL" (right-left)"at the end of every stereoscopic media name (images and videos) depending of how your stereoscopy is realised.
The editor and the web player cannot render stereoscopic content, you need to manage with raw stereoscopic content divided into two. This means we recommend to import a monoscopic version of your video into the sequence editor first, then position your hotspots, image or text using the WYSIWYG to finally replace the source of the 360°media by the stereoscopic one. See animation below :
NB: Stereoscopic content is only support with projects you've copied manually on phones. Not for online or downloaded projects for the time being.