That depends on the clearance in front of the drivers and the excursion they make. To make sure that the accordion surrounds of the two drivers can not touch each other you might want to use a spacer ring between the drivers. I.e. an 18 mm MDF ring between the drivers doesn't hurt the outcome but might not be needed.
Yes, I'm curious about the performance of the 12" in the Cubo 15 Extra Extended because it has a lot more excursion. Also, I have a 15PS100-8 to test out the box.
With a removable circular baffle for the 12" to fit the and seal the 15" box opening.
It's not going to do great, do to its very low Vas (even compared to other 12" drivers). Its Vas is low, especially for use in a 15" enclosure, causing a large dip between 60 - 100 Hz. The PS100 will not have that problem (as its Vas is 3.5 times greater).