Gail...I am a little late to this discussion...but I have Cee Cee...my three foot long eastern female in a 30 gallon tank, and I am getting ready to put another large female in with her. Cee Cee has plenty of room in her enclosure, and when Seeley was in there with her, they were fine in there together. I have Seeley in a 29 long...he is two and a half feet long...although he isn't very big in diameter, and I am getting ready to put another male in with him. I have Hermes and Possum...both a foot long...in a 29 long, and I am getting ready to put a third male in with them. You should be fine putting another female in with Zena. Worse comes to worse and you feel like they don't have enough room once they are together...go on Craig's List and find a nice used tank for them that is bigger.

To add to the above discussion about small garters...
After seeing how small eastern babies are and talking to a breeder...I would never want to breed easterns. The babies are just so small. Not something I would want to deal with on a regular basis.