For example, the recorder is 720P HD and supports 4 channels, which are channel 1, channel 2, channel 3, channel 4.

Channel 1 and channel 2 are a pair, so does channel 3 and channel 4.

1. If 2 ordinary analog cameras with different resolution such as 1000TVL and 1200TVL are plugged into channel 1 and channel2, they will be working.

2. If 2 HD cameras with different resolutions such as 720P and 960H are plugged into channel 1 and 2, only one can work and the other one can't.

3. If one ordinary analog camera such as 1000TVL and one HD camera 720P are plugged into channel 1 and 2, there is no doubt the 1000TVL camera won't be working and 720P will be working fine.

To sum up, the 2 cameras plugged into a pair channel such as  channel 1 and 2(or channel 3 and 4) should be in same resolution if they are HD cameras. If they are not HD cameras, it doesn't matter.