I wanted to do exactly the same thing, but as far as I can tell you can't. I'd also really like for the snippets to be properly nested, such that when you finish the second one, it carries on with the rest of the first.
Right this is not possible right now. You can only envoke another snippet after you reach $0 (which is reached after you tab past the last placeholder), or you break out of the snippet mode by moving outside the snippet, etc.