This may be obvious, but I can't find a simple way to do this. I added a layer of building outlines, created the models, then cut out two adjacent lots as per the guidance notes in the Philadelphia example. I then pasted these back in, created a new building and generated models. Everything works fine, except when uploaded as a web scene they exist as separate layers. To enable split-screen, slidebar etc it looks like you have to have the as-built and redevelopment models added as sublayers to the building layer.
Does anyone know how I can do this? It's simple enough to create a layer, but not a sublayer, nor to assign a layer as a sublayer to an existing layer.
I might be misunderstanding what you mean, but if you want to prepare to layers for comparative view in a 3D-scene (aka "Swipe view"), then all you need to do is give them the same Group Name in the export settings (see the section "Layer Group" here).