If you only have one video source cable, but multiple devices to hook it up to, then you can use an A/V switchbox, A/V distribution amplifier, or a coaxial splitter.

An A/V switchbox takes multiple composite or S-Video inputs and channels them to one output, while a distribution amplifier does the opposite (takes one input and channels it to multiple outputs). If video is coming from a device with only one A/V output, and you need to get it to a TV and EyeTV, then these devices could help. A switchbox can only route to one device at a time, meaning you could use your video signal with EyeTV or another device, but not both at the same time. A distribution amplifier can pass one source on to multiple devices simultaneously.

A coaxial splitter takes one cable or antenna line and distributes the signal over two or more outputs. If you have a cable modem, you can split your signal so that half goes to EyeTV, and half to your modem. If you use cable with your TV, half of the signal can go to your TV, and half to EyeTV. Due to the nature of cable/antenna signals, you can watch a separate channel on each split line (assuming a cable box isn’t involved).