(define-minor-mode A-mode
  :global t
  :lighter " A"
  (if A-mode
      (B-mode 1)
    (B-mode 0))) ; hooks here are added/removed...

(add-hook 'A-mode-hook 'B-mode)

(progn
  (setq foo 10)
  (print foo)     ; 10
  (let ((foo 42))
    (print foo))  ; 42
  (print foo))    ; 10

(put 'A-mode 'B-mode-enabled (bound-and-true-p B-mode))

(unless (get 'A-mode 'B-mode-enabled)
  (B-mode 0))