Lots of different combinations work for different reasons, sometimes with stuff you'd never think would go together. You just have to try stuff and see. Last night I took about a third each of St. James Woods, Cult BRM and Sutliff Strawberry Delight and set them out to dry a bit, cut the Woods up into smaller pieces and mixed them well and stuffed all into a meer, then set it aside to marry while I ate dinner. The after dinner results were grand! Far more depth than any one of them alone.
I've used up to four in a bowl, one for the very bottom to take all of the tar and to be more or less thrown away, and one for just on top to take the charring light, and two in the middle 80% for the real smoke. Sometimes it is just to use up bits and pieces of a smoke that I don't otherwise care for or to use a more flavorful tobacco to spruce up something with less flavor. On another thread someone just found that out mixing 10 Russians with Pirate Kake.
But if you think about it, many commercial blends have many tobaccos in them and I've made up my own blends too that had scraps of many old complimentary tobaccos I was trying to use up, so what is the difference? But I enjoy the layers for how each tobacco's different cut compliments the next and how the flavoring changes as you burn down through the bowl.