Has them try to "prove" who the real Lucy is.

"So here's the run down," he grinned at the two "Lucys." "One of you isn't the actual Lucy, you just believe you both are. It's a little trick I pulled. See, the real Lucy desires to please me above all else. Why don't you both try and decide which of you is really Lucy among yourselves?" he was grinning throughout his explanation, knowing full well that this little contest would have them both desperate to please him.

"Of course Master, I'll show you I'm the most submissive slave girl you could ever ask for!" Erza declared, sitting up on her knees with her hands behind her head to jut out her boobs for Natsu's viewing pleasure.

"Please, I'm the real deal, watch!" Lucy stated, crawling up to Natsu, before moving to give his cock a lick.

"Did Master give you permission to do that?" Erza taunted. "A good slave should wait for their Master's say so, even in matters of pleasuring him."

Lucy actually blanched at that, and protested to Natsu, "I'm so sorry Master, I got overzealous!"

"Don't worry about it, you can continue, although she is kind of right," Natsu chuckled, making Lucy happily engulf his cock in her mouth, and Erza pouted.

"Master, may I join in?" she asked.

"Sure you wouldn't conflict with her?" Natsu teased.

"Not if it interferes with the pleasure we give you Master," Erza asserted.

"Go ahead then," Natsu nodded, and Erza crawled over, and began to lick his scrotum.