You can use another master cylinder, as long as all the dimensions fit alright. Dorikin has written about this before. But if you want an AE86 master, I have a spare, but I wouldn't be willing to part with it for a few weeks til I am sure my new brakes work. If you can wait let me know.
The only advantage of dual master cylinders I can see is that if one fails you can still brake alright with the other one. Or possibly you can run different size pistons for front and rear, although I've never seen it.