You can use multiple EyeTV units on one Mac simultaneously. They can be all of the same model, or different EyeTV products.
This is an unsupported feature that has some bugs. It will not work in all circumstances.
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.
You can use it experimentally with the latest EyeTV software, but we won’t be able to help you if things go wrong.
Products that have dual tuners built into to one device (EyeTV Diversity, EyeTV Netstream DTT, EyeTV Netstream Sat, and HDHomeRun) 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.
More than one EyeTV HD unit can’t work on the same Mac at the same time.