Does someone have experience from this?

If I book a RTW for two people travelling together they will end up on the same PNR as far as I understand. If I after ticketing want to change travel dates for one of the persons, will they charge me? Technically, it's only a date change which I am not supposed to have to pay for. On the other hand, since they will have to split it into two PNR's I'm assuming they will have to re-write one of the tickets. So, do I pay the change fee or not?

Strictly the ticketing and the reservation are not the same thing - I very rarely have a one-to-one mapping between the two.

Since you don't need to reissue the ticket for a date change I can't see that there could be any charge - at the most your local OneWorld office would sticker the ticket and I have never even bothered with that.

Ack! No, no, no. Call "them", and let them know that you need a new PNR built. If "they" won't do it, call Qantas or go to a travel agent. Make sure, though, that they cancel out your part of the itinerary on the first PNR, or it will auto-cancel when you miss your first flight. Could also cause problems with double bookings if any of your flights overlap. Finally, they might have built two PNRs to begin with, you need to make sure what sort of reservation you have.

Hagbard, a reroute is called a reroute because you are changing the routing of your ticket. If you don't change the routing, you don't need a reroute. These things are too much of a pain in the arse to go about rerouting willy nilly. When a reroute is done, other than the date of travel when things change a little, you can't really just reroute one part of the ticket. Or "rewrite" as you called it. The closest thing that they can do is a sticker endorsement. Otherwise, if one segment needs rewritten, grab a couple of mojitos because you will be there a while.

This is why it is always a good idea to book each traveller on a seperate PNR.

Not always such a good thing. Let's say you both want to upgrade. With most airlines, separate PNRs will mean the person with highest status will get upgraded first and the other person may never make it. If they don't and you try to downgrade to join them, you may well find your original seat allocation is also gone. Messy.