That's a question i've been asking myself from quite a while, i'd be buying both just to achieve that monoblock which to me is pretty unique and neat looking.

Now i know that the motherboard's chipset aren't the same:
ASUS ROG MAXIMUS IX EXTREME - z270
ASUS ROG MAXIMUS X FORMULA - z370
Although i've been doing some researches and i found out that in some cases, they can work properly if the screws fits in the right place etc.
Can any of you confirm they're "compatible"? i've already emailed the asus support to ask them directly, but others opinions on this would be really appreciated!

My plan would be to have a custom loop on the MAXIMUS X FORMULA, but with the CPU block of the IX EXTREME, as that's so far the most beautiful ROG themed waterblock i've been able to find, and sadly it sticks with the motherboard.
Anyhow, if some of you are wondering why would i do that, it's simply because i care about both the looks and the oc performances, as well as the fact that the z270 chipset doesn't support the i9 9900k ( CPU i'll be running ).

Thanks for your time, i'm looking forward to sort this out!