I want to use multiple EyeTV units (or dual tuners) on the same Mac

You can use multiple EyeTV units on one Mac simultaneously.  They can be all of the same model, or different EyeTV products.

This is a preliminary, unsupported feature that has some bugs.

The only real limitation is that while you can watch and manually record live TV from multiple Live TV windows, automated recording is not perfected.  EyeTV will pick the first available unit to record to; there is no way to select which EyeTVs will record.

In the future this feature will be perfected, and then it will be announced.  Until then, you can use it experimentally with the latest EyeTV software.

Products that have dual tuners built into to one device (EyeTV Diversity, TVDuo) work as expected - watch two separate live channels, recording one or both if you like.  There are no problems with using both tuners that are resident in one of these devices.