Hello,
I have a question before I answer. What kind of data are you populating into dropdown from Form library 1.
Create a field and get only the selected values in Form 1 into this field and autopopulate the Dropdown box in the form 2 with this field. In This way you will be showing only selcted values in Form 1 in the dropdown box of Form 2
I hope this will help you.
Kalyan G Reddy
Infopath MVP 2007 and 2008