That works fine if you are the only significant contributor to the project.
If you start getting significant contributions from people using the GPL version, dealing with the dual license can become problematic. Best for you is if you can convince the contributors to assign they copyright to you. That lets you then fold their contributions, and continue dual licensing the result.
You'll first have to make sure that the contributors own the copyright on all the code they contribute, because they cannot assign copyright of something that isn't theirs. Remember, your contributors are working on the GPL version, so they might copy/paste useful things from other GPL projects into their contribution without thinking about it, because that is fine under GPL. They might not stop and think, "wait a minute...this is going to a dual licensed project that requires copyright assignment, so I can't bring in outside GPL code!".
That's in the best case, where your contributors are OK with copyright assignment.
You might also start getting contributions from people who are not willing to have their code go into the commercial version. There are two ways you can deal with that.
1. Maintain two branches in your project, one for a GPL-only version, and one for a dual license version. If you want some feature in the dual license version that is in the GPL-only version via a contributor who won't assign copyright, you'll have to implement it yourself. You can use the implementation on the GPL-only side for guidance for things such as algorithm selection, as long as you are careful to not do anything that would infringe the copyright on the GPL code.
2. Reject those contributions.
The risk with that approach is that if those contributions are useful enough, your GPL version might get forked, and people who don't need the commercial version will choose the forked GPL version over yours, because it has better features.
Probably best to go with the branch approach. That way you are more likely to remain the primary source (no pun intended) for the GPL version. There will be some contributors who are OK with dual licensing, and with the branch approach you get their contributions and can ask make your pitch to them for dual licensing. If some fork becomes the primary source, you are out of the loop, and will likely never find out what contributions to that are from people who would dual license to you.
