[Xy-pic] double arrows

[Davide Sangiorgi]

I'd like to use double arrows in the pictures of a talk.
I tried drawing the double arrows as below, therefore using
\ar at 2@*
or
\ar@{=>}@*

\[
\xymatrix at C=1pt{&   *+[]\txt{A}
\ar at 3@*{[|(4)]}[d]
& *+[]\txt{C} \ar@{=>}@*{[|(4)]}[d]
&   *+[]\txt{E}& *+[]\txt{F} \\
}\]

In both cases, the distance between the two parallel lines in
the double arrow is thin. Is there any way of making this distance a
bit larger? (Ideally, the same distance as the two external lines in
a 3-arrow  defined using \ar at 3@*)  ?

Many thanks,
Davide