Back when I had a boat, I had a pair of Cannons' that I really like, I only fished 1 other person so didn't need to stack.
When I was guiding up north I fished 4 people on 2 Penn 600 which were also good downriggers but required more maintainance, I would stack the lines 20 feet apart, 1 thing I would point out is that the bottom line probably caught 70-75% of the fish. I personally have not used Scotty downriggers, but most of what I've heard aboutthem has been positive. :p