Hi,
Thank you for the detailed explanation.
Currently, Helix Ultimate does not support a WordPress-style child template system, and we cannot confirm whether this will be added in a future release.
For your use case, the safest approach is to keep one official Helix Ultimate installation and create two Joomla Template Styles. You can place style-specific CSS under Template Options → Custom Code → Custom CSS, as this field can be configured separately for each style. However, the physical custom.css file and template override folders are still shared.
For different markup or layouts, you would need to use Joomla alternative layouts, page-specific classes, or conditional logic based on the template style or menu item. If completely separate PHP overrides are required, duplicating the template and manually tracking upstream changes through Git or a comparison tool remains the practical workaround.
Your request for child-template inheritance or truly per-style CSS and override folders is valid, and it can be considered as a feature request for the development team. However, we cannot provide any commitment or timeline at this stage.
Best regards.