Ok, Vixen shouldn't be too hard to work...
you'd setup a new sequence like you're making a new song.
you get to the plugins part, and add the Pixelnet Dongle, and set it for the com port, and whatever channels (1-4096) if you were using that many channels.
to add the 2nd dongle, you just add another Pixelnet Dongle, and set it for it's com port (different than the other one), and whatever channels you were gonna use on that one. (the channel count will start with the channel after the last count of the first dongle, so the 2nd dongle would control 4097+, if you set the 1st dongle to use all 4096 channels)
and just repeat for whatever other dongles you have.