Small correction: Does not dose.
I think
Our goal when creating this series was twofold.
is a very good interpretation. It really depends on the overall context, but this one seems pretty good to me, as it stands.
The difference between
- "Our goal in creating this series was twofold" and
- "Our goal creating this series was twofold"
seems minor. 2. seems more causal or informal than 1. Sentence 2. is a grammatically incorrect version of sentence 1. since I believe there should be a preposition before the word creating. However, it still sounds natural and clear to me. I can easily imagine a native speaker saying or writing this, and it is understandable.