Just as asio4all but better.
Yes I know, it is Windows, blocking us, or something else.
Since so many years.
I was using an UR-RT4, perfect but no ADAT so not so perfect, and I need more inputs so I bought another interface, but I miss the RT4 RN preamps.
So some evening I dream of a cubase 11 allowing this in W10 xxx.
Some intelligent abstraction layer 
Well the ASIO drivers arent made by Steinberg. They are made by the respective audio interface companies.
I guess Steinberg could come up with a new ASIO driver SDK, but that would require every company writing new drivers. Coreaudio uses some extra buffer that I guess helps with the clock drift and other problems synchronizing completely different hardware that operate with different converters and latency.
I don’t think that will ever happen. Unless Microsoft suddenly would adopt ASIO into the OS, or come up with something better, that again would require new drivers… nahh.
Buy interfaces that can be expanded, even if it costs a little more in the beginning.
It wont be that easy. ASIO is based on hardware interrupts from the audio device. So many cards will end up with many hardware interrupts. While for example mac audio system uses timer interrupts that can be shared. Some RME drivers can share io’s but I think they do that with only one hw irq.
some interfaces are multi client ASIO
they can build one big ASIO device
Multiclient ASIO means they can run with multiple ASIO programs simultaneously, not they can run multiple ASIO devices.
RME introduced the ability to use up to 3 devices together with it’s multi client driver back in 2010, I think
that’s why I remember that…
I don’t know how they call that feature
and there was a ASIO server introduced by Steinberg years ago, to enable the use of more than one ASIO device at a time
but it has no Win10 support and is out of active development as far as I know
AFAIK not even RME allows the use of aggregated devices of their own series (anymore). You can run for example several devices, but you have to sync them via cable. And Totalmix only runs on a per device basis. Two devices using the same driver have to be switched in the control panel to change settings. Unfortunately I can not check that myself at the monent.
Sure you don´t mean the “multiclient ASIO Driver” which has never been officially supported by Steinberg.
maybe that was it… never used it… just heard about it
I used my old Digiface and Multiface together for a Live Recording years ago (2008 I believe)
build an aggregated device
but low latency was not possible and was not needed for the live recording
and Nuendo did some strange things during the recording related to the waveform drawing
switched off the waveform drawing during the break was the trick to bring down the load of the system
I saw so many Mac users dealing with aggregated device just before the show starts
ending up with a stereo recording provided by the house guys
or a second computer doing the multitrack while the original Mac did the Waves Multirack or something similiar
and it always worked before
Give Voicemeter Pro a go, worth experimenting with:
Ideally I would have my 2 or more interfaces connected to the same PC and Cubase seing all of them for inputs/outputs.