Not explicitly dark, but the implications of what they've done are dark.

"Think they'll know?"

"Who? Their husbands, their co-works, their family and friends?"

"All of them! Think they'll have any idea that we're not actually them?"

"Not if we keep to it, they won't. We learned enough about them, and besides, a head injury allows for serious leeway on what we can say and do. Oh, I don't remember this person? Memory loss, the injury. Oh, I just flirted with a guy? Oh, sorry dear, things got a little messed up, I'm so sorry!"

"God, we could get away with so much! I wonder if we can get away with sleeping with some random guy and pass it off as just a brain glitch?"

"I know I'm going to! Have fun with it! These lies, they'll swallow them before the actual truth, you know."

"Well, yeah. Who the hell would believe inside these lovely bodies are the brains of two old men?"

"No one! They'd sound insane for even suggesting it! So, let's have as much fun as we can!"