Having 2 DD at diiferent secondary. Is this madness?

Which has the best learning support department? Which will give dd2 the best chance to reach her potential?
Which external agencies does dd2 see? What do they think she needs from a secondary?
Dd1 being miffed is an also-ran. She'll get over it.

(I have kids in two secondaries. Different school holidays are a pita, but life goes on. My youngest has some additional needs. I would rather she be in the right setting for her than pay too much regard to whether a sibling was miffed about the placement.)

The rivalry isn't relevant, they will just have to get over it.

Add for the travelling, presumably your dd1 will be in lower sixth next year, surely she will be mostly spring herself out? So the only problem is whether you can get dd2 to/from the further school, is there a bus service or a safe walking route? She should mostly make her own way there anyway.

My mother had my brother and sister in separate single sex schools, and a car. It drove her up the wall.
It's better to have both at one school if at all possible. I was eleven years behind my sister, and comparisons both positive and otherwise were made. That is inevitable. One set of uniform is much simpler. Besides, going to the same school will keep siblings in touch with each other. It also makes home-school communications and vacations easier.