Elon Musk took another step in combining his electric-car company with his solar-energy company, announcing on Monday that the two had reached a $2.6 billion agreement. Independent members of both boards approved a deal in which the car company, Tesla Motors, will acquire SolarCity for stock, valued about $2.6 billion based on last week’s average price, a Tesla blog post said. SolarCity shareholders would receive 0.11 Tesla shares for each SolarCity share they own. As…