On Fri, 06 Jun 2008 07:13:21 +1000, Herodotus
wrote:

I have
two main water tanks and at present I can't see how I can lead output
to an intermediate without all the lugging up the companionway or
installing an extra pump. Space is also an issue here.

Are the two water tanks isolated from each other? If so, run the
output of the watermaker to just one of them, and use a transfer pump
to fill the second tank if you are satisfied with water quality.