You cannot add a wait step except to a transition or a campaign.
One way to handle this is to have the stream run on different days like T/F. Another way is to have two streams: One sends Fridays, the other Tuesdays. When an Email is Sent, the lead is transitioned to the other Stream, so it ping pongs back and forth.
Another way would be to add Programs with wait steps, but that may be harder to manage.