I haven't introduced any fancy UI elements or on-screen guides for this yet, so
1. LeftShift + LMB while in the Platform Editor to select multiple platforms
2. CTRL+C to copy those platforms
3. CTRL+V to paste those platforms
4. Drag them off of the original platforms.
4/22/21 Patch: Multi-platform-select and Platform Copy/Paste
Source: Steam Community (opens in a new tab)