Having had a night to sleep on the idea, I realised why there is a reason no common simulation software would let you even bother with this...
If you have 2 drivers, sharing a rear chamber, but each with different front chambers, then you are creating a situation like having 2 drivers with different characteristics sharing a common chamber. It's usually a bad idea.
I would not consider that goldwood 12" or the dayton 10" options considered to be much of an improvement over the current pyramid drivers. I don't think I would waste money on either for a functioning 18-70hz sub.
If a bandpass box is not producing useful box gain over a traditional vented or sealed box, then it's being implemented poorly. Properly designed bandpass is a way to achieve greater box gain within a desired bandwidth than possible with simpler box alignments. 6th has a better chance of producing useful gain and wider bandwidth than 4th order. That said, with the ability these days to freely simulate various horn loaded, tapped horn, line and tapped line options, often the bandpass box options are passed up. There are cases where within box size restrictions, a bandpass can still offer the greatest box gain with an acceptable response even over more sophisticated box designs... It's all about finding the level of box sophistication that fits the driver and box size limits set.
How much amplification will you have for this?