I am you have these surnames in your file because they are somehow related, Your paternal and maternal lines, your wife’s paternal and maternal lines, etc. Splitting these lines into different file will end up resulting in more work for you. Some people will end up in more than one file. You will have to keep track of them and make updates to multiple files. No matter how carful you are things will get out of sync, updates made in one place won’t get made in another.
Using Silverfox’s methods it is fairly easy to split of a branch of your tree. This allows you to limit reports and charts to a portion of your file or to share a branch of your file with someone.
Using file merge to put multiple trees back together is much more difficult and prone to error. Think carefully about why you want to split your file into multiple trees.