Woman from Another Planet by Frank Belknap Long

On Earth a man named Peter Summers and his wife Ruth -- two people out of Earth's teeming millions, but chosen with care because of their exceptional courage and comeliness of form -- went about their daily affairs with no memory of what had happened to them on board a Martian ship a short month previously.

Summers was a writer of mystery novels and his wife an artist whose paintings had been widely acclaimed by critics. They were quite ordinary people in some respects, brilliantly exceptional in others. They had both been used as models by Tragor in constructing experimental android men and android women, but those particular androids had been discarded as not quite what the Plan called for. The android who resembled Summers and the lithe-limbed, sultry-eyed robot-woman who was almost an exact duplicate of Summers' wife had not been destroyed, however. They had been kept immobilized in a laboratory on Mars, the disks on their thighs turned off.