At first glance, there's nothing all that similar between Realty Income (O -1.67%) and Walmart (WMT 0.22%). One is a real estate investment trust (REIT), the other a retailer. But there's an important intersection, since the properties on which Realty Income is focused are largely occupied by retailers. In fact, Walmart is one of the REIT's largest grocery store-related tenants. A comparison between these two related companies is worth close consideration for long-term dividend investors.Walmart vs. Realty ...