If I understand the question, you are asking for a way to add multiple classes in anchor links in Markdown.
As per the README of Goldmark the Markdown processor that is currently turned on by default in Hugo:
Currently only headings support attributes…
## heading ## {#id .className attrName=attrValue class="class1 class2"} ## heading {#id .className attrName=attrValue class="class1 class2"}