Try doing it slowly. Play the D, pause, then the D again. Concentrate and feel how you need to blow it on the alto, to get it stable and clean. Think about lip pressure, position of mouthpiece (angle/depth), finger pressure, breath support... Relaxing. How long can you hold the D steady. Then do the same for the B. Then go slowly from one to the other. Count five between each, thinking about what you're doing. It should be there soon. Then speed up. Watch your fingers as well. You could be lifting them in sequence instead of together.
I find alto more challenging than tenor, which is such a lovely relaxed blow. But I've played nothing but Alto since Xmas and it's making a huge difference.
Back to tenor this weekend for me, repad should be finished by then!