How would that work? I think you posted something on the github but I don't think what you're conceiving here is possible with this API. It just reroutes HTML insertions from the template to a different location, there is no duplication.
<template for="placeholder2"> <?marker name="placeholder1"> <?marker name="placeholder1"> </template>
<template for="placeholder1"> <?marker name="placeholder"> <?marker name="placeholder"> </template>
<template for="placeholder"> Here is some <em>HTML content</em>! </template>
Before you couldn't make this dangerous (without JS) because there was no way to auto expand the templates
<div> Here is some <em>HTML content</em>! <?marker name="placeholder"> <?marker name="placeholder1"> </div>
(only the first matching marker set is replaced.