Clarification regarding design of a device.

Hi, I have say a crypto device and if I pass a data stream assuming echo "test"

/dev/aes and when I read it I get an encrypted output. Now if a program opens the same device twice should and pass different streams should I differentiate those 2 streams and have encrypted buffers of these 2 streams as output? If yes how should I differentiate it? Or should I not differentiate and provide different devices say aes0,aes1 or should the userspace program worry about locking and sharing of resource?

Thanks.