Where are you missing,,,,what distance are you shooting.
A true double tap for anything longer than a couple yards is very questionable in terms of accuracy.
I have no idea what level of proficency you have with a pistol, please don't take this the wrong way,,,,,when the first shot fires are you perhaps closing your eyes with the firing of the pistol???
Beyond just a couple yards you should have one sight picture per shot....once you get accustomed to following the front sight, you'd be surprised how fast you can fire shots and get a good quick flash sight picture.
Ever shoot USPSA or IDPA.....Listen to the good shooters pistols when they shoot arrays of targets, the really fast guys have a steady/smooth rythmn like a sewing maching,,,,,,,,Smooth beats choppy speed and accuracy-wise.
You'll hear bang,,bang,,bang,,bang rather than bang,bang,,,,,bang,bang,,,,,bang,bang
Check out:
http://64.62.172.100/~brianeno/index.php
for lots of pointers....This is Brian Enos' forum. Brian is a Grand Master USPSA shooter and there is loads of good information on his site. It is geared mostly toward USPSA and IDPA with of course the majority of time devoted to USPSA, but there are lots of good tips applicable to whatever type of pistol shooting you're into.
Brian along with several other very good shooters post regularly and are very willing to help.
H4444