Adding a new Russian tort

Question
Hey there. So I have had 1 Russian for about a year now. I still can't tell the sex. It is about 4" long. I recently bought another Russian, also with an undetermined sex and close to the same size...maybe a tad smaller. The new one is RUTHLESS! She goes nuts and constantly bites my first born. They have plenty of space and hiding areas but it will chase the other anywhere it goes. Even when I put them outside their pen with no barriers, it will chase the original tort everywhere.
Interestingly, when I give them a soak together, they seem to love one another. No biting. Why is that? Vulnerability maybe?

So I am just wondering if the nipping/biting is ok. Will they get used to each other? I want them to be friends so badly! Also curious as to if this behavior could answer the sex determination of the torts.
Also curious if there are any ways to KEEP a higher humidity level in the pen, besides wetting some of the substrate? It dries immediately and the peat moss/sand mixture is extremely dry and dusty.
Sorry for the lengthy e-mail but never thought turtles could stress me out or make me feel sad. I am such a dork!
Any advice would be much appreciated. Thanks!

Answer
Nipping/biting is not OK. There are several possibilities here. The tank may be too small (aim for about 100 square inches per inch of tortoise, so 2 4" torts should be in about 800 square inches- about 40"x20" or so.

The newcomer may be a bully, or be very stressed or nervous in the new habitat, etc.

I am sorry that I do not have a rock-solid answer for you on this. You can separate them for a while and try again, try a bigger pen, or just see if it goes away... it just sort of takes some gentle experimentation.

As for humidity- peat/sand is not a substrate I would recommend- instead I use long-fibered sphagnum moss (like Mosser Lee brand at Lowes, etc. http://www.mosserlee.com) It holds moisture very well.

You can try a mix of clean soil (top soil, mulch, potting soil, etc. just no vermiculite, perilite, etc.), sand, and some sort of organic material to hold moisture (sphagnum moss, shredded bark, coconut fibers, etc.). Adjust the mix with less sand for more humidity, or more sand for better drainage.

http://www.tortoisetrust.org for more info.