Glad you found TCS. I sure hope we can help!
Going #2 on a cold hard surface is often a sign that there's actually a health problem. Not always, but often. What makes me think it's not a stress-reaction is that she keeps going in the same place. A stress-reaction is more often expressed in peeing outside of the box, and most often on something that smells like the person that comforts kitty, like a couch, the bed, or laundry.
I would do a couple of things. I think a trip to the vet for your female would be in order. Bring a sample of her #2 with you.
Are they both spayed and neutered? Poop is often used as a territory marker. This would be more of a predisposition thing if she's not spayed.
In the meantime, if it is not health-related, I would do a couple of things:
1) Purchase Feliway - either the spray or a plug-in. Feliway is a synthetic hormone that mimics the "friendly" markers in cats' cheeks and helps lower stress. http://www.catfaeries.com
. This isn't the cheapest place to purchase it, but I post the link because I would also consider
2) Using Flower Essences. For this situation, I'd go with either Territorial Rescue or Calm and Serene. Flower Essences are not as universally effective as Feliway (which doesn't work for all cats - but it does work for most cats) - but the cats that respond, really respond.
3) I'd add at least one more litter box to your home. She's maturing, her personality is more developed, and she may becoming "pickier" about her bathroom. Some cats do not like to use the box if there is something else in it. Some also develop a preference for pooping in one box and peeing in another box. Just FYI, the rule of thumb to help cat owners prevent problems like this is to have one more box than you have cats. For people in apartments, it can be a problem. But I would consider at least one more box.
4) As a safety net, I would purchase Cat Attract additive to add to the litter. There is a "Cat Attract" litter - it is expensive - but I also hear it doesn't clump very well (though it works REALLY well for cats not using the litter box). However, there is a product developed by the same vet, and you add it to your existing litter (so the clumping still works). It worked really well for a kitty of ours that was peeing outside of the box due to a stress situation. http://www.thecatconnection.com/page/TCC/PROD/LTR-7551
The most important thing to remember is that about 85% of the time, a cat that pees or poops outside the litter box - that was previously using the litter box - has a health problem. Cats are great at otherwise disguising the fact that there's a problem. But they are creatures that operate "by association" - so if they come to associate the litter box with pain (in urinating or pooping), some of them will stop using it.
Hope this helps,