Make sure you create your form using InfoPath and not a List Form. List forms can pull data from multiple lists, but can only submit to one list. To submit to multiple lists, you need to create a full-fledged InfoPath form (as opposed to a simple list form) and use qRules with multiple mapping to submit to both lists. Here's a link to a video Hilary did showing how to do it:
http://www.youtube.com/watch?v=WftCp_EbcCU