I doubt that a twitch will work, very hard to compress sintered stainless steel.
One of the thread sealing compounds would be worth a go, I use a fair bit of Loctite 577, its about the only one rated for potable water. Will cope with temperatures over 100oC...
I would lightly sand both surfaces to get rid of any existing glue, apply a thin coating to both sides and press together, might be worth hot curing to get maximum strength.
Mind you a tube of 577 is about the same price as a new air stone, if you haven't got any use for the rest of the tube you might be better off just ordering a new one.
Mark