Cannot --allowsigned multiple items

Context: bare metal installation.

I tried to set --allowsigned for both 'cpp' and 'extern' in multiple ways in configmgr:

--allowsigned = cpp
--allowsigned = extern

--allowsigned = cpp,extern

In neither case was I able to constrain both cpp and extern. In the former case it appears that the last declaration of --allowsigned "wins"; in the latter, I think the first item "wins" (though I could be misremembering).

How can multiple items be constrained?