If SquirrelMail will take a CSV file, make two copies of the file: file1 and file2 (and keep your original).
Open them both up in a text editor (Notepad on Windows, or TextEdit on Mac).
In the first file, delete the last half of the lines and save the file.
In the second file, delete the first half of the lines, but save the header line, if it has one that says something like “Name, E-mail” and then save that.
Now Import one file at a time.
Let us know how this goes.