OK, we get it. Rod Stewart and Stevie Nicks are both classic rockers. But their differences are more striking than their similarities, something that was certainly true last night at Quicken Loans Arena, the first stop of the summer installment of their “Heart and Soul” co-headlining tour. Even when they performed together during the middle of Stewart’s set, each appeared a bit out of sorts. Nicks wore sunglasses as she and Stewart sang his “Young Turks,” and Stewart, who didn’t seem to know the words, sang only a few verses with Nicks on her “Leather and Lace.” Still, the two seemed to really appreciate each other (Stewart referred to Nicks as his “dear friend”) despite their differences and each delivered a strong performance.