Hi Alex,
When you use a Send To Desktop action on a transaction, the line items are sent with the transaction automatically. You do not need to add additional Send To Desktop actions for each line item. Keep in mind that if you attempt to send a transaction to QB that does not have any line items, the transaction will not be posted and I believe you will receive a split line conflict.
Your order should resemble something like:
1) Create the transaction record
2) Create the line item record(s)
3) Send the transaction record to QB
I hope this helps.
-Audisho