Who dunnit?!

We're having doggie drama. We inherited Lewis's sister, Gracie, this summer while Brian's mom worked long hours. They are 5 years old and best best friends. So, it's really hard to imagine splitting them up. But, after the last week, we might just give both of them away. We've had mysterious messes left all over the house and no one is owning up to them.
Lewis, the loyal, valedictorian in his class, social disordered dog that he is would hold his bladder til Jesus returned before he would go inside. Or so we thought.
Gracie, is very, um, pretty. She's like the popular girl at school. Loves to go to the party, is the life OF the party, but not a whole lot going on upstairs, if you know what I mean. Gracie will go potty pretty much anywhere. Just because she has to go.
Until this point, we were totally sure that Gracie was the one leaving "presents" for us all over the house. But then the runny #2's started popping up. First in the "man" room, (which will someday soon be converted into the nursery). Then at 4:30in the morning last night, in our bathroom. I let Brian take care of that one since I had to get up at 6:30 this morning. Then the culprit let it go again right AFTER they'd gone in the bathroom and outside between the hours of 4:30 and 6:30 am.
So now we're trying to catch them in the act. I'm thinking security cameras, alarms on the carpet, anything. This is unacceptable and I did not get dogs to live in a poop factory. Here's pictures of the two suspects. See if you can decide who the guilty one is.
Gracie and Lewis, look nothing alike but from the same litter

We didn't do it! It was Daddy!! We promise!

Pssst. Mom. It was Gracie. She's so stupid, I kept telling her to hold it. Please don't be mad at me! I know I'm your favorite! :)