2 up on one bike vs 2 smaller bikes - real world costs

[Bobduro]

Hi Guys,

The missus and I are planning to head off from London to Oz in 2010 and are looking at biking options.
Now this may well be an impossible question to answer (how long's a piece of string?) but not including the cost of the original purchase, what would the real world difference in cost between travelling 2 up on a bigger bike (i ride a 990 adv at present, but would probably get something cheaper) and riding 2 smaller, cheaper and more fuel efficient bikes such as an XT660's.

I know that the cost of trips in general is massively dependent on how luxurious/frugal you want to be but I'm guessing the actual 'biking' costs are fairly set regardless....

If anyone can shed light on the realistic cost of such a trip that would also be great. We're thinking that £10k should see us through but maybe we're being totally unrealistic.