Just the same way like in English you can put any last name in the plural to mean "the family of ...", like the Smiths, the Lennons, the Bushes, so you can do that in Russian, and that's what you need to do when addressing a couple. In your case, you can write Ева и Виктор Ивановы. Still, putting last names into plural and declining them is a tricky thing when dealing with Russian, Rosental has a whole chapter devoted to that. What is the most important, is that last names (rather rare ones, though) that end in -аго, -ако, -яго, -ых, -их, -ово and (Ukrainian, rather widespread) that end in -ко (-енко) are not declined, and their plural is the same as the singular, that is why Ева Черных and Ева и Виктор Черных, Виктор Петренко and Ева и Виктор Петренко.