If you are thinking of just stacking two together back to back, you won't get double the notch depth. You would have to separate them by at least a 1/4 wave of coax, taking the velocity factor into account.
PAR Electronics has a good faq about this.
Note that the inexpensive Radio Shack FM traps do not have much of a notch below 100mhz, so rewinding the input coil would help.
The PAR Electronics filters have served me well. Instead of trying to stack two questionable cheap filters, why not just use one quality PAR FM filter?