You didn’t specify what Mac and video card you have installed…
Sounds like you have a video card which has one DVI port and one ADC port, or one DVI port and one VGA (analog) port.
The newest Macs come with video cards with 2 DVI ports.
If your Apple Cinema displays are the newer ones with the metal bezel and DVI connectors, you will need one ADC to DVI adapter or one VGA to DVI adapter, depending on your card.
Dont get the 99 dollar Apple DVI to ADC adapter, which does the opposite of what you need (connecting older plastic bezel Cinema Displays with ADC connectors to DVI cards)
Belkin makes this adapter, I think there are others…
https://catalog.belkin.com/IWCatProductPage.process?Merchant_Id=&Product_Id=92397