defcustom and :set functions dependencies

[hjuvi]

Hi,

I'm trying to define some customization variables, which are dependent
on one another.

(defun setf1 (param value)
  (if v2
    ....
(defun setf2 (param value)
  (if v1
    ....
(defcustom v1
  :set 'setf1
(defcustom v2
  :set 'setf2

My problem is : symbol v2 does not exist when setf1 is defined.
If I move both defcustoms before defuns, then symbols setf1 and setf2
are not defined when compiling v1 and v2.

Is there anything like a declaration before a definition (like in C
language)?
Are any other way to solve this?
Thanks.