Thanks a lot wwmjkd for your reply. I haven't separated them yet. They are still staying together.

When you say it didn't look a typical lock, I was wondering if it was because I was trying to taking them out and when I noticed they were mating, I put them down immediately and took the picture. I want to show you some more pictures here.

This is the picture I took on the 29 May. I saw them coiling together. I found some pictures on the net saying that it was a sign of pre-mating behaviour. Is that right?



Here are some pictures I have just took 30 mins ago.
First of all, I have noticed that their bellies have turned pink. Both of them shed not long ago and I don't know why. This one, particularly, turn quite red.





Next, I want to show you the tails of them. Is that possible to tell their sex?

Lu's tail





Gil's tail





Both of them (bottom - Lu) (top - Gil)