If both filters approximate a magnitude of $1$ in their passbands, and a magnitude of zero in their stopbands, then the resulting filter will approximate a magnitude of $2$ where their passbands overlap, a magnitude of $1$ where the passband of one filter overlaps with the stopband of the other, and a magnitude of zero where both stopbands overlap. Note that FIR filters cannot have a constant magnitude (unless they have only $1$ filter tap), so all constant values are just approximated.