The second woofer is unnecessary; you don't need it for power in a home environment and it's only giving you an additional 3dB of output so I'd lose it. The space devoted to it coud be reassigned to the horn, allowing either a smaller box overall or a longer pathway for an even lower Fc. Don't underestimate what you can accomplish from a small box. This SPL chart is my Tuba 18, a 45cm cube driven by an eight. It doesn't go as low as your design but it sure goes a lot lower with more SPL than anything else in its size class. You can get what you want in a far smaller package or get even more from the same size box by bagging that second woofer.