The X6 400 cm and 430 cm masts combined.

In the first list you'll see the exact results for the possible combinations of the X6 400 cm/X6 430 cm pair - and in the following list the most important of the same results are shown in a little more well-arranged way (I hope).

A few comments:

Length.

No problem here, no matter how you combine the two masts, the results are the same (approx. 416 cm).

Stiffness.

The "parent" masts have close to the nominal stiffness (19 and 21). The combined masts both end up with a stiffness around 20 (the 400 bottom/430 top being the stiffest).

Bend Curve. 

For most mast series it generally holds true, that the smaller the masts, the relatively softer the top - and conversely the longer the mast in a series you grab, the relatively stiffer in the top is. Even knowing that, the difference in bend curve between the 400 mast and the 430 mast is surprisingly large (a little more than 2 points). Taking a quick look at the line-up it seems that it's the 400 cm mast that shatters the otherwise pretty harmonic picture of the X6 line-up.

Combining the two masts, the 430 bottom/400 top comes out a little more Flex Top than the 400 mast, making it useable with Neil Pryde sails.

The 400 bottom/430 top ends up with a bend curve around 15. This combination will probably not fit smaller high end Maui Sails sails, and Neil Pryde sails will probably lack a little looseness in the top/profile in the bottom with this combination - but the combination will probably be very good for sails from most of the other brands.