Black & Tan

I’m getting ready to order a couple of AG kits. I’ve pretty well decided on the Dry Irish Stout for the first and am considering quite a few options for the second. I got to thinking it could be fun to pick something that would work with the stout to make a black & tan. Ignoring color, what’s it take in a beer to make a black & tan. Do I just have to make sure the second beer is considerable denser (higher FG) than the dry stout?

I’ve poured it with Harp lager and with Bass ale.
My problem is, when I’ve tried to make black & tan with home brews, they just blend together. I wondered if it was density or if it has to do with the nitrogen in Guinness…?

this is a black & tan I made with a bourbon stout & light ale braggot.

not sure if it was the carb levels that kept them separated, but the stout was much thicker & had a higher OG and FG than the braggot. the braggot had much lower ABV. but most times I also have problems lately keeping my home brew black & tans separated as well. no matter them serving temps. wish I could be more help than that.