I read that knew you must be talking about LBC. I haven't tried their mix yet, but maybe the next time I stop in. I think you would just pour them both in and let them mix. The black and tan layering would only work if your milk stout was on nitrogen.
Draft 1: Lightly smoked Weizen
Draft 2: Belgian Wit
Draft 3: Rauchbier
Primary 1: German Scwarzbier
On Deck: Belgian Brown Ale, Pale Ale, Stout