I also considered pursuing a fellowship after already being out of pharmacy school for a year or 2 as a practicing pharmacist. Every fellow I brought that up to at the time looked at me incredulously and urged against it saying I was overqualified and would be seen as a step back. They didn't mean that I was overqualified in the sense that I had more relevant experience, but pretty much consistent message with what Hels pointed out - the program REALLY isn't intended for practicing pharmacists. When you graduate with a PharmD you're not only a healthcare provider (after licensure) but also qualified to pursue a health scientist route.
An industry fellowship is specifically intended for fresh grads who want to develop into a health scientist straight out of school. Companies typically sponsor a fellowship for the purpose of grooming new grads from the ground up to be future leaders in an organization (preferably theirs), not to serve as a transitional program for existing pharmacists. Even from what I've seen, in the same department at the same company the fellows get a different induction, and get exposed to more events, strategic and high profile projects than regular entry-level employees. They get preferential treatment, and usually get offered a higher position/advanced role after their 2 year fellowship than someone who joined as a regular entry level FTE at the same time. It doesn't end there - they're often in a faster promotional track throughout the first decade of their career, especially if it's with the same company. As I said before - they're not "put to work" so much as they're "groomed".
Additionally, sponsoring a fellowship is an investment decision to the company with delayed (in some cases, intangible or unrealized) return. The actual cost to the company per fellow is quite a lot more than the fellow's stipend. That means that it's a risk and someone had to do some convincing to get the company on board with this. So - you can see why a company would consider accepting a pharmacist as going out on a limb when it probably doesn't fit the candidate profile that they sold to the decision makers when getting the fellowship approved, and there is no shortage of ideal candidates at Mid-Year. Simpy put, it's a lot of money and time to abandon the principle of who the fellowship was set up for.
Dual board certification in itself would probably not make a big difference because those achievements (respectable as they are) would be more related to being a healthcare provider. While being a US-licensed HCP is a plus, companies are primarily interested in your qualifications as a healthcare scientist. What they would prefer to see is that you've managed to develop some transferable skills as a health scientist while working as a pharmacist, and are qualified to apply for a regular position. To apply for a fellowship at this stage might come off as not having made any effort to explore and develop as a health scientist beyond what you graduated with - although you've been out of school for years.