Have read about the pouring devices for pouring a Black & Tan. Apparently some device is needed. Just tried pouring a Black & Tan without a device and it didn’t work, the Imperial Stout and the Imperial IPA mixing right away. What device do you use?
The reason a Black and Tan works is from the difference in gravity of the two beers. Having two Imperial beers may be problematic.
That said, you can use a spoon. The actual device looks kinda like a turtle.
More than just “kinda like” a turtle…
Yep.
I agree, however, Guinness is app. 4.2% and Bass is app. 5%. So as long as the “tan” beer is less gravity, in theory, it should work. Get the terrapin, that’s the best way to pour them.
Still thinking about it, would it matter so much the abv%, or the final gravity?
Are you sure the nitro pour has nothing to do with the separation? I always assumed it did.
I’ve done it out of the bottle with a spoon: Smithwicks and Guinness. I prefer Harp.
Some Guinness bottles have widgets. 
Dogfish does them at the pub with 90 minute and Chicory Stout - no nitro.
Like others said, there needs to be a gravity difference (final gravity I’d wager) to keep the separation. Also, if the stout is higher gravity and you pour it on top, it will just sink though the IPA no matter how gentle you are.
I pour black and tans from bottles with a spoon all the time. I have the lager head as well and that’s fun. I am not sure it is about the relative gravities. I have made a balck velvet with dry cider and stout. The cider should be much less dense as it was a much lower finishing gravity but the stout floated just fine.
I think it’s mostly about the spoon/device that allows the dark beer to spread out and not mix too much at the interface. It sill mixed some though.
I moved this to “General”, probably not safe to assume that only All Grain brewers know how to pour Blacn and Tans.
I bent a larger head spoon to fit in the glass and flattened the head. It works great. As far as what makes one float and one sink, all the responses are correct. It has to do with ABV and gravity. But you’re probably not going to know which has a higher gravity. Especially if you are using imperial beers. So I typically just go by % ABV and it works 99% of the time.
Final gravity. Lower FG will float on top if poured correctly.
It helps to really foam up the 1st beer (usually Harp). Give it an inch or more of head. Then slowly pour over the top. My dad had some new gimmick that is a disc that sits on top of the pint with holes on the side that trickles the stout slowly over the top. Worked great for the 1st one, but when splitting 2 bottles to make 2, the 2nd one was not as good. The conclusion was to stick to the turtle spoon.