You still have to catch an HQ fish with your first mooched fish to be able to double mooch, so it's a pretty rare occurrence.

I've seen someone catch a Titanic Sawfish with HQ Goby > Mooch > HQ Wahoo > Mooch > Titanic Sawfish. The unfortunate thing here is Silver Sharks will sometimes share the mooch with the Sawfish. There are speculated to be factors such as time of day and weather that help narrow down some of the rare fish. Some people have concluded that Sawfish can be caught shortly after noon in game time in clear weather off the above double mooch.

Lots of good fishing discussion on disciplesoftheland.com btw.