This is one of those questions where the answer is "it depends". It depends on whether you're a startup or established business, what your goals are and what your user's expectations are.
The most common approach is to tackle one platform first (generally iOS), work out what your v1.0 of your product actually is, solve any issues and potential problems, and then have the other platform fast follow and eventually catch up making it more of a translation exercise. I've seen this approach work well in the past for both design and build.
If you have a project in mind that you would like to collaborate on, I'd love to hear about it!