It's not just about Webmin, it's about the underlying system. You can't have multiple versions of PHP at the same time in one action, especially with modules — it's not just about how to run
PHP can be used with apache as a module, or with a CGI interface. You can't have two PHP modules at the same time; you can have two php-cgi binaries, but they must have different names/locations, different locations for PHP modules, different locations for includes, etc. You have to build them manually and satisfy all the dependencies and structure. It's not that easy and It's not going to be easy to manage with virtualmin — you need to specify the PHP version for every virtualhost manually in the Apache configuration file, as SeeDeoubleYou did here: Multiple php versions simultaneously on Ubuntu.
You can easily run only one version of PHP with Webmin/Virtualmin, but not multiple.