I like the idea, but I don’t think this can be done with the Reply Template theme component. The theme component creates the button automatically for you and doesn’t supply any place that a second button could be added. You will need to add two reply templates to the topic in order to get close to what you are looking for.
I realize this is an old question, but the topic was bumped a few days ago. I’m not sure where things were at in May 22, but you can now add either a list of one or more users to send the PM to by using either the recipients
or usernames
parameters: Reply Template.