Why the hell won't my black and tan's stay separated? Need help from a Bartender!!?!

I've been trying to pour the perfect black and tan. I'm using bottled Bass ale and bottled Guinness draught. I am pouring into a tall 22 oz. beer glass. I've used everything from a bent spoon, a funnel and a spoon, I bought a Brutul turtle and nothing seems to work. I've even resorted to using a syringe to pour over the turtle which worked awesome for a Black and Blue, but thats one time out of a dozen tries. This is my sequence:
I open the bass and pour it into the glass with about 1-1.5 inches of head. I mix the guinness and open that and then pour very slowly over the turtle or the spoon. It's sort of separated, but it only lasts about 2 minutes and you can see the guinness mixing with the bass. After about 3 minutes it's totally mixed. Why the hell is this happening. Should I just stick to pints? Is guinness in a can different than the bottle? Need Help from a bartender!!!!