Q: How to connect the old apple cinema HD 23" display and an external thunderbolt hard drive at the same time?

Yes, if the disk drive has two Thunderbolt ports. Plug the Thunderbolt display to the computer.  Connect the drive to the Thunderbolt port on the back of the display.  Connect the MiniDisplay Port adapter to the other Thunderbolt port on the drive.

Well, old Apple Cinema Displays doesn't have thunderbolt ports on the back, there is only fw400 and usb 2.0 there.

Connect one Thunderbolt port of the drive to the Thunderbolt port on the computer.  Plug the MiniDisplay Port to DVI adapter into the other Thunderbolt port on the drive.  Connect the display to the Mini DisplayPort adapter.

If the display uses ADC (Apple Display Connector)

<http://upload.wikimedia.org/wikipedia/commons/0/09/AppleDisplayConnector.jpg>

you will also need a DVI to ADC adapter.

<http://www.amazon.com/Apple-DVI-ADC-Display-Adapter/dp/B00011KHT2>