You can smooth the combined response in Xsim using the "System" under Curves and apply smoothing to it.Could I ask how it is that your individual responses are smoothed and your combined one is not?
Have a look at this project for ideal crossover slopes:
https://www.diyaudio.com/community/...tbcd-gb-dxt-sb-acoustics-sb15nbac30-4.391310/
scroll down for the woofer/tweeter responses!
https://www.diyaudio.com/community/...tbcd-gb-dxt-sb-acoustics-sb15nbac30-4.391310/
scroll down for the woofer/tweeter responses!