According to the narator "according to shipwreck rules" they must decide jointly which island they will choose. Where was this rule when Richie and Lucy arrived together in 2006 and made individual choise which island to go to?

The announcer also says the rules have changed...
Richie and Lucy had make seperate decisions, so did Rory and Anna.

Anyhow, they can changes the rules from series to series as it is their programe!

what if both don't give in could be there for hours.

I read in the rules that if they can't decide they flip a coin that has S for Sharks and T for Tigers on it and which ever side it lands on, they join that team.

Yeah I think the voiceover on last weeks Shipwrecked said when Mike and Matt were making their decision 'Half an hour later...'.

Sure The Almighty can change the rule but must notify us about these changes because they go on about shipwreck rules. Thanks some one for reminding me about Anna an Rory.