The Mosaic doesn't use crossovers to emulate a 12 string's usual tuning [high E & B strings unison; lower four strings in octaves]. So it should be easy enough to cover the octaves & unison on every string. The Tone control's targeting effects might be something you'll have to do outside the H9.
There's a [F]actory preset in the HarModulator algorithm called ELEC12STRING ROO. It's a good place to start. Dry + unison + 1OCT, with offset delays to simulate doubled strings. I happen to like a little more 'octave' in the mix, and the cyclical modulation there is a little too deep and regular for my tastes.
I went with 2 pitch shifts an octave up, a lighter mix, and slightly longer delay times. Modulation is envelope controlled, and shallower (but spread out: up & down 3 cents). The expression pedal maps the Mix from 25-50%, and the Mod Depth from -3c to -8c, for a deeper variation under foot control.
The HarModulator, Diatonic, and QuadraVox algorithms may be the best bets for more natural 12- and 18-string presets. The H910 / H949 is perfectly serviceable for any of this, and my first choice for 'glitchier' alien 12-strings. There are several other algos to work with for variations that include reverb, or more exotic & 'unnatural' 12-strings.
