So 3 people are saying different things. I did the whistlewind scarf upgrade to star and dismantle...didn't give me accel sash.
Split Infinity FAQ says "Buy 6x Accel Sash from one of the stores, and then equip two of them per party member. This will give you Lucky Stagger auto-ability, which allows you a low chance of Staggering the enemy instantly regardless of its current CG value."
No offense, but Nash is wrong... I've also never heard of an accel sash, probably because I am playing the NA version... if you buy the item that is 10,000 gil, you don't even have to upgrade it. You can simply buy two of them and put them on one person and get the random:stagger synthesis ability. Moreover, if you upgrade them, they still belong to the gestalt synthesis group and therefore would still work.
Speed Sash > Energy Sash
-Items that belong to gestalt synthesis...
Survivalist Catalog, Hunter's Friend, Speed Sash, Energy Sash, Champion's Badge and Axis blade / enkindler
There is no item in the US version named Accel Sash, that's a translation from the Japanese version.
The item you're talking about is either the Aurora Scarf, which you obtain by using a Rhodochrosite on a Starred Whistlewind Scarf, or a Speed Sash, which you can upgrade to an Energy Sash by using a Mnar Stone on it after you Star it.