There is some strange behaviour I'll have to investigate further with weapon swapping, but Icestorm appears to share an unleash count between weapon swaps, so you can't swap between two Whispering Ices to get a double unleash. Non-item skills seem to reset their count to 1 seal.
Edit: Okay, we've found the reasons, if the same skill is socketed in the same socket position in the item (or is granted by the item) then it won't reset seals when you switch, so they're effectively shared. We're going to fix skills starting at 1 seal when socketed or switched to, just in case there was something funky you could do with that.