Conversions, conversions! You could represent the twin-linked missile launcher by a big targeting lens or something where the second weapon would be. You just have to explain to your opponent what you've got. I try to hold myself to a tournament-standard wysiwyg and tournament-standard of painting (i.e. I don't use unpainted models) but I obviously can't hold anyone else to that standard since I'd hardly ever get to play if I did.