Male/female is supposed to be the best combination for jealous dogs so you've got that going for you too. I don't think it matters which lead you use; if they are equally happy with the Y lead; I've never tried one.
Hopefully, it will just take some time. I feed Lucy in her crate; otherwise, Ringo would have eaten all the food. Your oldest probably does not like it when she bothers him while eating; try and put a stop to that if you can. Give your first dog his own space too; a place he can go to get away from all the puppy shenanigans when he is tired. He is not in non-stop play mode like your baby.
Tonight, I had 3-4 toys on the flloor and just started throwing one for Lucy and one for Ringo; it usually gets them started to play together.
Good Luck and I hope they learn to really enjoy each other.
__________________  Mommy to Lucy, Ringo, and Matthew |