We just recently had wills drawn up to be sure our children went where we want them too. We did not have to name each other as guardian but it is written something like 'in case parent is not willing or able to care of the minor children then "so and so" is named guardian".
I agree with the other posters, have a long conversation with the person your considering. Some people will say yes, off the cuff because they love you and your family. That is not enough though. You need to be clear with how you hope to have your children raised.
We chose my nephew, who is getting married this year. We sat down and talked to him and his fiance' and were clear that should the marraige not last, that we would want the kids to stay with him. Again, he is just getting married so they do not have kids yet...we asked long term things - Are you REALLY up to loving and caring for older children that are not yours when you have your own?
- Where do you plan on living? It is important to us that while our kids are still young that they have a similar upbringing/enviroment. Ideally, we would like them to stay in the same area and complete elementary school where they are. YES, it is asking alot, but their lives would be changed so drastically w/o us that we want to keep them in a familiar enviroment (fiance' is not from this area so it was very important to us to know where they want to raise their family).
It is alot to think about. We brought the subject up to them and asked them to think about their own questions before we met to discuss it and they came with some good questions and reassurances of their own.
Good Luck