The X6 460 cm and 490 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.

The 460 bottom/490 top happens to be a little shorter (approx. 474 cm) than the 490 bottom/460 top option (approx. 477 cm).

Stiffness.

While the 490 cm mast has a stiffness as expected, the 460 cm mast shows to be somewhat softer (IMCS 23.3) than promised by Neil Pryde (IMCS 25).

No matter how you combine the masts the stiffness ends up around IMCS 26 (funny - that's probably the stiffness you'll obtain, if you extend a X6 460 mast with the promised stiffness (IMCS 25) with approx. 15 cm).

Bend Curve. 

Both "parent" masts have bend curves in the 15-16 area - but when you combine the masts, they end up having very different bend curves: The 490 bottom/460 top combination shows a very Flex Top bend characteristic, making this combination almost useless (or an "emergency solution") - even for the most hard core Neil Pryde sailors.

The 460 bottom/490 top combination, on the contrary, might be a very sensible choice for sailors with other sail brands - perhaps even Maui Sails sailors (although perhaps not ideal).