Hi, I am new hear, but i am pretty current with bleach.
The only reasonable explaination, maybe this would shed some light on the subject, is that, possibly Isshin was or is a member of the Zero Squard (the Royal Guard).
If i am reading through the manga and other sites correctly, that would put Ishin above Soul Societys laws and the issue of 2 Swords having the same ability. If that would be the case, then he could posses an indetical Zangestu, or another sword that is like Zangestu with similiar abilities.
Along with the other posts, it is possible that families of shinigamis may have the same or similiar techniques. On bleach.wikia.com, Ginrei Kuchiki sword is stated: "The name of Ginrei's Zanpakutō is unknown, but its appearance is greatly similar to Byakuya's Senbonzakura, with a rectangular guard and white hilt.".
So it may not be exactly the same sword, but might be possible the techniques can be simliar. I know the movies don't correctly follow the story lines, but in the Diamond Dust Rebellion, they did prove that 2 exact swords could exist, but they said nothing about two different swords (with two different names) could not pocess the same techniques.
I do agree with another poster however, in that it seems that almost anything can happen.
To better explain my position on why i think Isshin might be part of the Royal Squard (Zero Squard) is when he was fighting the Grand Fisher, Isshin states that all captain-lever shinigami consciously keep their Zanpakuto to a managable size, otherwise everyone would be swinging around swords the size of skyscrapers.
In all the episodes, were other shinigami referred to managing the size of their swords, all were referncing swords closer to Ichigo's sword size in it's shikia state. No captain has ever said that they managed a sword down to a small size from something as big as a sky scraper.
Well, Thursday will be the release of manga episode 406, hopefully we will learn more about Isshin.