The X6 520 cm and 550 cm masts combined.

In the first list you'll see the exact results for the possible combinations of the X6 460 cm/X6 490 cm pair - and in the following list the most important of the same results are shown a little more well-arranged (I hope).

A few comments:

Length.

There's a difference of almost 4 cm between the 520 bottom/550 top combination (approx. 533 cm) and the 550 bottom/520 top combination (approx. 537 cm).

Stiffness.

The "parent" masts have close to the same stiffness as stated by Neil Pryde.

Both bottom/top combinations are placed in between the parent masts - the 550 bottom/520 top combination being the stiffest (approx. IMCS 34) by approx. 1 point.

Bend Curve. 

Both "parent" masts have bend curves in the 13.5-14.5 range, but remarkably, the longest mast has the relatively softest top.

Combining the two masts, the 550 bottom/520 top option is probably only usable for Neil Pryde sails, while the 520 bottom/550 top option will probably fit nicely to most other sail brands (probably including Maui Sails sails).

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A private note: The last mentioned combination, which certainly shouldn't be optimal with Pryde sails, was used by me the whole 2008 season. When my X6 550 broke in the bottom section I decided to use the top from this mast with the bottom section from my 520 mast, and it turned out to work OK. Please have in mind, that as the combination was used in a RSR 10.7 it had to be somewhat extended - and as you'll see in the "extension-section", extensions happen to turn the masts a little relatively softer in the tops.