I have two projects at google code. I would like to have the first linked to the second, it means, in the second project I have a folder that points to the first project. When I run svn up at the second folder, the folder that points to the first repository must be updated too.
What you are looking for is the svn:externals property. Let us assume the projects are 'project1' and 'project2'. When you check out the trunk from project2, you want a subdirectory called project1 containing project1's trunk. Here's what you need to do:
svn propset svn:externals 'project1 https://project1.googlecode.com/svn/trunk' https://project2.googlecode.com/svn/trunk
svn:externals is a versioned property, so it will generate a commit.
You can just as easily apply the property to a working copy and commit it. See svn help propset, in the svn:externals section.