When it comes to the two men, Engels generally acknowledged Marx as the philosophical force and helped to disseminate his ideas. Not to say that Engels didn't have any of his own. By the time the two had met, he was already a rather notorious Revolutionary in his own right with publications of his own. But once the two men started to work together, he spent more time disseminating Marx's ideas than getting out into the spotlight himself.

In that time, he was widely acclaimed and not at all forgotten. His portrait often hung in Bolshevik offices next to Lenin and Marx's and would be on floats in May Day parades. Later, he became something of an unfair scapegoat for people who wanted to "save" Marx but repudiate the Soviets.

But in mass media, "Great Man" type history tends to sell. So Marx moved to the front and Engels to the back. As a result, he tends to come up less in a more modern sense.

I would also note that Engels wrote on perhaps a broader variety of subjects than Marx did. For instance, his famous work The Origin of the Family, Private Property, and the State is a seminal work within family economics and anthropology, but as those areas of academia don't tend to animate vast political parties that take over large Western empires, those types of works tend to get overlooked.