I am a british citizen marrying my american fiancé within the next 3 years. She wishes to move to the uk but keep her u.s citizenship hence have dual nationality, likewise for me I wish to remain in the uk but obain u.s citizenship and be a dual national also. How do we do this?

If she is a US citizen, there is no restrictions on her part to move so she can settle down in UK as long as she wants, so she has to start with PR and then become naturalized... it's the same with you in case you two decided to move to US... marriage, obtain permanent resident status, become naturalized.