Two pillars of Elon Musk’s empire are facing financial crunches as the entrepreneur seeks to combine the two companies through a controversial acquisition.

Tesla Motors Inc., which makes electric cars, disclosed in a securities filing Wednesday that it has to pay $422 million to its bondholders in the third quarter, and that it will raise additional money by the end of the year. The purpose of the additional capital, among other things, is to support its proposed merger with home-solar company SolarCity Corp. Mr. Musk is...