I'm perplexed , making a black and tan is much harder than it seems - yet the guiness refuses to sit on top

I've tried so many times to make an authentic B & T and I even have a pour over turtle thing for pint gles , but the density of the beers just doesn't want to cooperate with me. Guiness is supposed to be less dense and float on the B, the same with something like murphy's irish stout and harp. These mix pretty quickly. Interestingly enough, I've managed to make one successful type that I made up -- It's sam adams ale and Rasputin imperial stout (heavy beer) ; it makes a nice Tan and black with no mixing. Can anyone fill me in on the mystery of the true black and tan.