max_upload only deals with the size of the uploaded files, not the entire POST object (all form fields PLUS files). In addition, there are also timing issues. You'll want to increase the time php will stay awake to process the request.
upload_max_filesize
post_max_size
max_input_time
max_execution_time
The larger the files, the longer the input_time and execution_time need to be to allow the file to be uploaded, and any sort of processing you are doing on the php end. If it takes 10 minutes to upload a file, the input_time/execution_time need to be at least that long (php default is usually only 30 seconds).
That only deals with the php end though. You will probably also have to adjust relevant settings for apache/nginx or whatever you're using for a webserver. Those settings should be slightly larger than the corresponding php settings.