When is Tempest going to wake up and see that 2 heads are better than one when the warfighting gets complex?
F35 seems to manage single pilot. It really depends on how complex the mission and how much you want to spend on through-life costs by doubling the numbers of your most expensive manpower category. If you are going to use 2 x pilots you are likely to be over-training one of them (assuming one will be on WSO duties while the other flies), and if you want a WSO then you need a separate training pipeline for them. Manpower costs are significant in any programme - part of the equation that led to the Wedgetail buy will have been the manpower saving of 6 people per crew (vice E3D) over the next 10 years.
Given that Tempest may be manned or RPAS or a combination of both, it would be reasonable to assume there will be some form of near-real time reach-back available. Whilst 2 heads are indeed better than one, access to several heads may be better still, especially if those heads can get to other resources that might not be available in the cockpit.