Hi Christian,
regrettably, you may not ensure this behavior in the web interface. You have two possibilities how to achieve your goals:
1.
You could develop a custom form control (
http://devnet.kentico.com/docs/devguide/developing_form_controls.htm) with two upload fields inside. You may add the validation according your needs in the code behind and save the content (files) programmatically.
2.
You could modify the BizForm web part (http://devnet.kentico.com/docs/devguide/modifying_the_code_of_standard_web_parts.htm) and check your custom file upload fields in the OnBeforeSave() event. You stop the BizForm processing (saving data) by:
this.StopProcessing = true;
Best regards,
Helena Grulichova