I have several problems planning an iteration for a single week. We use XP and TDD in Pairs and it is hard to decide who is pairing with whom. Is there any tool support for planning an iteration that also supports planning of pairs?
In XP you don't plan who pairs with who and when. Team members just pick a task to work on at the start of the day, in the daily stand-up meeting. If someone else also wants to work on the task, they'll volunteer to pair on it. Team members who do not pick a task first will pair with someone who did. Sometimes the person who picked a task will want to use the expertise of another team-member and so will ask them to pair with them. But that person may have something more critical to work on and decide not to. Pairs often switch while a task is in progress, especially if it takes more than a day to complete.
So, pairing up on tasks is fluid and informal.
The one thing to keep an eye on is that everyone on the team pairs regularly with everyone else. If that is not happening, it can indicate a deeper problem in the team -- someone not pulling their weight or people who do not get on, for example. If left to fester this can lead to siloed knowledge and key-man dependencies that increase project risk.
In those situations, some teams like to use "pair stairs" to keep track of pairing and highlight problems.
However, that is just addressing the symptoms (uneven pairing) not the underlying problem. It's better to address the problem head-on in a safe environment (e.g. run a retrospective) and work out how to solve it.