Over here in the states we call that a "Black and Tan" and is usually a
combination of an Ale and a Stout, with the Stout going on top. The layering
is made possible by two things (at least over here), one being the fact that
the Stout is on a gas system consisting primarily of Nitrogen, whereas the Ale
uses primarily CO2 (which I guess effectively creates slightly different
densities in the beers), and when pouring the Stout on top of the Ale you need
to use a spoon (Guiness has a special one that distributors provide the bar
with), similar to when pouring layered shots to keep the separation.
Cheers!
Josh@BarSim
www.BarSim.com
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