Do you have the HD Homerun model with two coax inputs or just the single input? If it is the single input that's the newer model and El Gato says:
Each tuner can be used by different computers at the same time, or both tuners can be used by the same computer simultaneously.
Try this, temporarily reduce all the complication you have going on: with only one
instance of EyeTV open on only one
of your wired Macs, are you able to record two different shows simultaneously?
My HD HomeRun has two tuners (both of which are connected to an OTA antenna in the attic)
Does this mean you have a single line to the single input, or are splitting the coax coming from a single antenna and running two lines to the HD Homerun or you have two antennas?
I thought I'd be able to watch EyeTV from two devices at any one time.
You should if your network is setup properly. I don't have an HD Homerun anymore, but back when I did, it was the earlier model with two coax inputs because I wanted one for clearQAM from cable and the other for ATSC. I was able to watch different channels on two different Macs at the same time, that's normal behavior for the Homerun so something isn't configured properly for you. One other thing to note is I never needed multiple serial numbers, I just used the same one on all the Macs in the house. This was in 10.6 and I gave up on the HDHR after that, going back to multiple EyeTV 500s instead.
I wonder if there is a bug somewhere preventing one of your tuners from being released properly, so any other device "sees" it as in use even when it's not, but first see if you can record two different shows simultaneously on that single Mac.
The most current discussion of the HDHR and Macs takes place here, George:
edited to add links to SiliconDust forum and El Gato's now feeble support