I'd imagine most 12-string plugins would just use pitch shifting and maybe chorus with it. But pitch shifting +12 semitones is going to sound bad. So chorus is probably your best bet.
Or track the guitar part once then use a capo and track it again. This can work when playing in some keys.
With that said, even cheap $100 12 strings can sound good, especially Yamahas. So you're best off getting one of those.
I just did a tune where I needed a 12-string type sound. I just played the part low and then played the part high, keeping in mind which notes would be played by wound strings, etc....