[Ben Ficklin]

"for a 2-seat aircraft"

There were some 2 seat versions built mainly to make it easier to train new pilots.

Originally the A-10s(CIA) and SR 71s(Air Force) were single seaters

[commish]

The SR-71 is a two-seat aircraft, the seats were in line not side by side, Front Seat was the pilot and the back seat was the ELINT officer. The training versions just had the back seat raised above the front so the trainer could take control of the aircraft as needed.

[SES1066]

Originally the A-10s(CIA) and SR 71s(Air Force) were single seaters

You are correct about the CIA 'A'rchangel being single-seat but the only built versions were the '12' versions; A-12, YF-12A and then the M-21 (launch platform for D-21 drone). The YF-12A was the USAF designation for the Interceptor version of this airframe and identifiable by the nose lacking the smooth chines of the other models to accommodate the Hughes AN/ASG-18 fire-control radar. However the 3 YF-12As were all two-seaters, pilot & radar operator. Two of the YF-12As were lost but the remaining one is at Wright-Patterson USAF Museum in Dayton, OH.

The 9 surviving A-12s are on display at the following locations;

A-12 60-6924 at the Air Force Flight Test Center Museum Annex, Blackbird Airpark, at Plant 42, Palmdale, California.
A-12 60-6925 at the Intrepid Sea-Air-Space Museum, parked on the deck of the aircraft carrier USS Intrepid, New York City.
A-12 60-6927 the two-seat trainer at the California Science Center in Los Angeles, California.
A-12 60-6930 at the U.S. Space and Rocket Center, Huntsville, Alabama.
A-12 60-6931 at the CIA Headquarters, Langley, Virginia.
A-12 60-6933 at the San Diego Air & Space Museum, Balboa Park, San Diego, California.
A-12 60-6937 at the Southern Museum of Flight, Birmingham, Alabama.
A-12 60-6938 at the Battleship Memorial Park (USS Alabama), Mobile, Alabama.
M-21 60-6940 at the Museum of Flight, Seattle, Washington.

[AFreeBird]

It was the A-12 that came first.

71s were always two seater. Recon officer behind the pilot. For the trainers, they raised the rear cockpit to give it a forward view.