Being a melodeon player I'm used to having an instrument that has two reeds producing the same note but tuned slightly differently to each other. In the dim and distant past I used to have an Eko 12 string that I loved to play (badly) until I lent it to a 'friend' who disappeared before giving it back. I always used to tune the strings to the same frequency as I thought that was what you 'had' to do.

Do any of you 12 string players out there tune the strings slightly apart?

If not, why not?

Where the strings are an octave apart does this hinder the effect or help it?

Do players of mandolins, bouzoukis etc tune the strings slightly apart?

Aren't piano strings tuned in this way too?