It isn't normal, but the first double I started working on was the flip about 2 months after starting to work on the axel. If you're working on the axel, it's a good time to start doubles because some people have an easier time landing doubles than an axel. Definatly try the sal, it's one of my weakest single jumps, but my best double, although the technique dosn't change when adding rotations a swingy jump like the sal can be easier as a double than a single. If you have a good loop, the only difference on the double is concentrate on keeping your skating side (landing side) long and strong. Make sure the rotation is around the landing side and the rotation is initiated by the hip and not the shoulders.