[austerity]

It's simple concatenation. The exact algorithm is described at the bottom of the page so that one could reproduce it (and their passwords) independently.

[spion]

What about taking the 128 MSB vs 128 LSB, is there any research into how secure that is?

[austerity]

I vaguely remember giving it some consideration. But the bigger point is that my judgement on these things is not to be trusted since I'm just an application developer and not a crypto expert.