Zacchaeus:
In fact, however, the situation you just described can indeed happen, according to my in-game testing. The additional effect of poisoning happens, the Pokemon's item heals the poison, and then the Poison Touch effect triggers.
If Poison Touch had raised the additional effect chance of attacks with an additional effect of poisoning the opponent, the result would have been inconsistent in the case of attacks with a different kind of additional effect, since even for these kinds of attacks, both the additional effect and the poisoning can occur with Poison Touch.
So, for example:
Poison Jab can have a 30% chance to cause poison, and Poison Touch gives its attacker a 30% chance to cause poison. Combined this results in a 51% chance (not 60%) to cause poison.
Crunch can have a 20% chance to decrease Defense, and Poison Touch gives its attacker a 30% chance to cause poison. The chance for both effects is 6%; that is, the Poison Touch effect doesn't raise the chance to lower Defense by 30% and thereby prevent Poison Touch from causing poison.
EDIT: Examples corrected.
EDIT 2: According to further tests, Poison Touch actually does have a 30% chance to cause poison. Editing accordingly.