Well, I don't know if I have this right, but it's worth a try.
I think first it has to be accepted that if you use a standard 50Hz crossover high pass and standard 50Hz crossover low pass target from REW, that the resultant combined outout will be flat as seen as the red line in the plot below.
50combined flat.jpg
So if we simply lift the low pass target with the simple house curve as shown, I would say the resultant would be as shown in the upper dotted line. It looks close enough to give it a try anyway. It gives you something to work with anyway. I see no reason to reduce the curve at 15Hz. Let the bottom drop as it may.
julien_50_house_combined.jpg
brucek