I was just struggling with this myself. It seems that you have to create a PhidgetVoltageInput for each input channel, and then use the setChannel message to attach each object to a specific input channel.
It also seems that once you've done that, the PhidgeteVoltageInput (henceforth - PVI!) objects ignore any further setChannel messages. I find that if I want to change the channel, I have to reinstantiate the object. (Though you probably wouldn't want to do this once you'd set the patch up. It would be useful for testing though.)
What I don't know yet, is whether or not that attribute is stored with the patcher (so that the next time you load the patcher the ports are found automatically), or if you need to loadbang setChannel messages to all of the objects...
... just checked - it looks like you have to send the setChannel message when you load the patch.And also if you're changing the default DataInterval using the setDataInterval message, you need to delay that bang by about 1000ms, to allow the Phidget objects time to attach (500ms was too short).
Hmm. Something I did made it all stop working. Had to restart Max & replug the Phidget. And it looks like I might need to increase the time before setting the DataInterval, as there's a tendency for the last PVI to attach to Channel 0 (which already has a PVI attached)... 2000ms seems to work.