Make either of 2 fields required but not both? Ive added a file field to comments so users can upload a file in a comment as well as post text. How can I make it so one of the 2 fields is required to be filled in to create a comment (so you cant submit blank comments), but without making both fileds required?
Check this thread and Conditional fields module.

If you're using D7 - the principle is the same as in the thread I linked. Basically you need to attach the name of your processing function to the validation array and there perform the checks.