no they shouldn't, look at the recent cup champs, Penguins split Kessel/Crosby/Malkin on different lines, Chicago didn't have Toews/Kane together and neither did the Kings with Kopitar and Carter. you don't stack 1 line because there easier to check, if they get shutdown you've lost. now on the PP that's that's different, and also have no problem for 3-4 shifts 5on5 loading up to catch the other team by surprise but for the most part 5on5 no they should be on different lines. especially Marner and Matthews