I was chatting to a guy at pod with a compound charged drag bike (hope to see them again this year, nice blokes) and he was explaining all the details of how he did his. I forget all the details now but he was saying how blowing one into another created a ratio of 3:1 for boost ie 1 bar +ve into the other turbo created 3bar +ve in the larger turbo. I seem to remember the reason being a small turbo will reach 1 bar (and thus create 3 bar total) faster than one big single turbo?

Thats all from memory though and I could have got my wires crossed