It was done on several occasions, like project "tip tow" that incorporated a B-29 and F-84s, "Tom Tom" that incorporated a B-36 and F-84s.
The Germans were working on a similar concept during the war with types like the Me328 coupled with an He177 or Ju388 and the Soviets had an interesting setup with their TB-3 carrying an I-16 under each wing.