Ace Double F-123 (1961): containing The Nemesis from Terra by Leigh Brackett, bound with Collision Course by Robert Silverberg.
The Nemesis from Terra by Leigh Brackett (cover by Ed Emshwiller, 120 pages.) [Magazine version was in Startling Stories Fall 1944 as "Shadow Over Mars." There was a British paperback with that title in 1951. ]
Rick Urquhart was going to tame and rule the turmoil-ridden planet of Mars. There was no doubt in his mind, for had not the old Martian mystic prophesied it? Power and glory appealed to Rick and shrewd determination ruled his every move. But then the idea didn't please anyone else; both the good and the evil forces at work on that planet had too much at stake. Rick was everyone's enemy and death stalked him from every corner. Even the woman he loved would rather slay him than expose a world to his dictates. But Rick laughed at their schemes, for how could the pattern of the future be undone by mere mortals?
(Bound with) Collision Course by Robert Silverberg - Cover by Ed Valigursky, 135 pages. [Avalon hardcover was in 1961; a shorter (?) version appeared in Amazing July 1959.]
After 4 centuries of space travel, man had explored and colonized every habitable world within a radius of 400 light-years. Not a trace of any other intelligent life had ever been sighted. It seemed as if the universe belonged solely to mankind. The discovery of another, more advanced race already busy colonizing far-off planets shook the very foundations of human feelings of superiority. But deflated egos weren't the only problem. Man was determined to continue spreading outwards, and the aliens obstinately had the same plans. What could prevent the inevitable clash of 2 stellar empires and the oncoming destruction to all their worlds?