Yes. Many IWW members are also members of other unions. They are women and men who form the fighting heart of such unions; rank and filers who strive to make their unions yield returns for the membership and not completely degenerate into docile pacifiers serving the boss.
Such workers have joined the IWW because they want to build a union stronger, more fearless, more honest, than the unions dominant today. Such a union must, in the end, match the unity of the employers with the greater unity of the workers, match the power of the employing class with the greater power of the working class.
That union is the one big union of the IWW.