Assuming Outlook. Create a new Contact Folder and copy the list to that contact folder.(Hold the CTRL Key and drag and drop, you should see a + sign as you do this.) Rename one of the distribution lists. Delete the first 50 from one and the last 50 from the other. (Select the first contact, hold the shift key and select the 50th to include all; those in between)Drag and drop the moved file back to the original contacts folder.