I wrote a couple of weeks ago about using more than one LP-16 in Linux. The music store where I bought mine told me that Apple might recognize more than one device. Is that accurate? I could be ready to switch to Apple after being a life-long PC guy. (I’ve used PCs since everything was in DOS and before we had Internet.)
In general, under Windows aggregation of USB audio interfaces can be achieved by installing ASIO4ALL. A (albeit slightly outdated description can be found here
But it is not entirely clear what you are trying to achieve in your setup.
Just more outputs? If so, there are various other routes you could consider as well.
First of all, uTrack24 (24 I/O) with USB, or dependent on your application you could also consider uTrack24 in a AudioLAN network allowing you to go up to 96 I/O.
I use the LP-16 essentially as a sound card with multiple outputs. I now have 18 audio outs for my virtual instrument(s). Of course, I also have more than 18 sets of amplifiers and speakers, and let the “air” do the mixing, though I tweak levels quite a bit. For live virtual instruments it is a more convincing arrangement than mixing it all down to two speakers. (More are always better.) I use this with a program much like Hauptwerk, but it runs on Java and is not limited to pipe organ sounds. That’s why it runs on PC, Apple, or Linux.
I prefer to do this in stages rather than buying a much more expensive device which also requires more expensive adapters like D-plug adapters, a new set of balanced cables, etc.
I am at the stage now where I’m dreaming/planning for the future, but I’m not entirely sure of what is even possible without spending many thousands of dollars.