Dear Trophe,
Two-seater conversions are more common now than they were during WW2. Back then, they sometimes removed radios or fuel tanks to cram a second pilot into any remaining space.
As early as May 1944, General Eisenhower ordered radios and a fuel tank removed from a P-51B (razorback) to allow him to personally survey D-Day beaches.
Many modern warbirds have armour, radios and rear fuel tanks removed to make space for after-market rear seats.
For example, the USAF only bought a handful of two-seat, dual-control TF-51 Mustangs, and most of them have disappeared. However, dozens of privately-owned P-51Ds have been retrofitted with second seats.
Similarly, only a handful of original Hawker Sea Furies were built with double Malcolm hood canopies, but several flying in the USA have been retrofitted with larger bubble canopies and second seats.
When I worked for Butler Parachute Systems, I packed a variety of custom-fitted pilot emergency parachutes for a bewildering array of warbirds.
Now several British warbird owners are retro-fitting their Spitfires, Hurricanes and Buchons (Spanish-built Me-109) with second seats and selling rides for thousands of dollars. Hurricane conversions resemble the Russian example, while Spitfire and Me-109 conversions are based upon wartime production patterns. Sounds like a good way to pay the (hellacious) operating expense and keep them flying.