Two of a Kind by Orrie Hitt

Cover copy reads: 'Men or women...her warped desires included them both'
An Explosive Novel Of A Beautiful Woman Without Morals and the Passionate Fools Who Loved Her

--
Sisters in Sin They shared everything including him. Frank Kane didn't know what he was getting himself into when he went to work for the Wilder Game Farm. All he knew was Lana, his boss, was the most beautiful, desirable woman he had ever met - that he had to have her at any price. There was another girl on the farm. Young beautiful Gloria who saw in him her chance for a normal life. She pleaded with him... 'Make me a woman, Frank. Make me a woman again and again. Make me a woman so that I know that it's true, so that I'll never have another doubt.'
----

First line:
''It was raining hard, very hard .'