Listen to the entire thing, I'm not tripping. 'Chris' starts out sounding like a boy, but even into the first segment s'Chris' clearly starts to sound like Tina. I never picked up on it when I was listening to the podcast way back in August (and I missed the joke Shawn made at the end) because I'd never heard Tina's voice before.
Jeff makes a joke at the end when Tina starts talking about something in terms of since she started working for Ziff Davis. It's along the lines of "I thought you were just here as a take your son to work day sort of thing", and she's like "lol shutup"/ Basically she broke character at the end.
And they never announced "Hey Tina is with us" throughout any segment.
EDIT: 6:30 - 8:30 minutes in it switches to Tina or something, but it IS Tina by the end of the show, although my new speakers make 'Chris/Sammy' sound a lot more boy-ish before these points, so maybe it wasn't her putting on a voice, and they just switched things without telling anyone.
...IT'S REAL TO ME DAMNIT!