You can download a template from one sub-account and then upload it to another.
To download a template, from the template gallery:
• Click on Options (cog icon)
• Select Download as → MJML file
Now switch to a different sub-account and go to its template gallery.
To upload the template:
• Click ‘Create a new template’
• Choose ‘By coding it in MJML’
• Select ‘Import MJML from file’ and import the MJML file you had downloaded
• When prompted, click the ‘Use drag-and-drop’ button
Related Reading: How to copy a template from your Primary account to a sub-account.