Yes you can, though not sure why splitting them into two streams is even necessary. You could just put all 6 emails in one stream and set up a cadence for the leads to get one email each week for 6 weeks.
If you really want to use two streams, however, you don't need a smart campaign to ensure that the leads are pulled into the 2nd stream once they exhaust the first stream. You can go into the engagement program, click on your 2nd stream, and edit the "Transition Rules".
Example of Transition Rule might be:
Smart List - Email is Delivered __________ (could chose the last email in the first stream that the leads will receive)
Flow - Change Engagement Program Stream (this should already be done by default by Marketo)
I'm sure there are other ways you can pull the leads into the 2nd stream, but that should work. Let me know if you have any other questions.