The caption with two girls had a deep impact on me. The idea of a secondary keyholder, which puts the guy an extra step further from any chance of getting unlocked, is quite intense. His sense of helplessness would be doubled... at least.
Interesting. I found that pic hard to write for ... I had it for quite a while before I came up with an idea that suited it. It had some interesting elements: the woman underneath is the one with the dominant facial expression; and she also seemed younger and more typically attractive. But they both have good eye contact with the viewer, which is always crucial in chastity captions.