If your worried about diversity, you can allow each Rogue Trader to a second carear as a 'specialization' that they can also buy advances from. Though to be honest, even a Rogue Trader doesn't have to have the Rogue Trader carear. (Like in star wars, a jedi doen't have to take jedi...) And if they do have the Rogue Trader carear, they don't have to be a Rogue Trader - they could be a free captian or even the RT's Ship-Master.
In my playing group, the Rogue Trader was nominated and elected before the game started, much to the (soon to be) Rogue Trader's suprise. But yes, there are a lot of way to handle multiple people wanting to the Rogue Trader class.