To elaborate on the science of double-dipping from a scientific perspective, we consulted a scientist. A GERM scientist.
"Germs can spread as long as there's a surface. When you have something that has a high concentration of bacteria, like a human mouth, you're going to inevitably see some transfer," said microbiologist and author Jason Tetro. "Usually that transfer is going to contain about 1% of the bacteria. So, out of the millions of microbes in your mouth, only about a thousand should be transferred to the chip. With even less bacteria being transferred back into the dip, if you use the same chip. There's a lot of dilution there."
So, I'm not a scientist (technically), but I have to assume this assessment works in my pro-double-dip favor, correct?
"Regular oral bacteria should not cause any problem, in this case," Tetro confirmed.
"But bacteria is not the only thing that can be transferred via double dipping," Tetro said, promptly hoisting me on my own petard. "Especially if someone is sick with a virus. This can be a twofold problem: You're exposing your body to something it might not be familiar with, so there could be an immune response. And if it happens to be a pathogen, such as the flu, cold viruses, or herpes viruses, this could potentially lead to an infection."
This is what scientists and science-enthusiasts alike refer to as the "minimal infective dose." Basically, you have to be exposed to certain number of the bug in order for you to get sick. In the bacterial sense, it's tens of hundreds of thousands of microbes. So that can't happen via dip, unless someone literally spits in it. But with viruses, it could be as little as one microbe, according to Tetro -- which unfortunately can be spread via dip. And even scarier, people can be contagious with viruses without even showing symptoms.
So, the verdict here is split.
Bacteria = never.
Viruses = definitely possible.
And that's not good for my argument, really. But it doesn't kill it. It's a mere dent in the armor of my ironclad stance.