Silly me, I thought this was about how to pour a black n' tan. I wish Guinness still had the old version of their FAQ online. It included a post I made in 1992 to the old newsgroup rec.crafts.brewing on density, boundary layers, and foam structure. I didn't even know about it, till an old friend in Wisconsin pointed me to it. Back on topic, the best beer float I have ever had is vanilla over Lindemans (sweet) kriek. About the only good use for that soda-pop beer.
Personally I don't get the layered B&T - if I want a layer of Guinness and a layer of Bass then I'll order one of each....