Howto fill out two CCK content types in one form

I am trying to get some advice on the best way to implement the following:

I am setting up a site to track my family and friends scuba dives. The way I am thinking about doing this is to have two cck content types:

1. The actual dive information
2. The dive location information

So far so good. However, here comes the difficult part. What I don't know how to do is to be able to have a user who wants to enter a dive enter the dive, and also the dive location using two scenarios.

1. If the dive location has already been entered by someone else, to select the location using a drop-down or something similar
2. If the location has not been entered, enter the dive location information directly from the form.

If anyone has general comments on the best way to implement this, I would appreciate it greatly.

Thanks.