Maybe I misunderstand your question, but it seems to me this is very simple to do with a couple of rules
Set up a seperate data connection for each of your libraries
Add a button to your form and label it submit, add a couple of rules to it:
Rule 1:
Condition: Dropdown = option1
action: submit using data connection 1
rule 2:
condition : dropdown = option 2
action: submit using data connection 2