Searching for the ability to use several (audio-)interfaces at the same time (i.e. CI2+, UR242, Gibson RIP, internal IO) a lot suggests using an ASIO-driver like ASIO4ALL. But this is for Windows. On a Mac with MacOS live is much easier 
. Just create an aggregate device. Follow instructions here Create an Aggregate Device to combine multiple audio devices â Apple Support (UK). Every DAW-software that uses the coreaudio from MacOS should be able to use this aggregated device, like Garageband, Logic Pro X and also Cubase.
Cost me a lot of searching before I found this out. Maybe it helps others saving some time 
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Hi and welcome,
I would say itâs worth to mention, most of the DAW vendors donât support this way officially (even Apple doesnât support it officially with Logic and GarageBand). Steinberg also doesnât support it officially.
Sometimes, there might be some timing issues.
If no central clocking of all connected devices Iâd say there will always be some timing issuesâŚthey may be imperceptible in many cases, but theyâll be enough to mess with phase in a multi mic setup for sure.
You can use one of the devices in an aggregate device as a âmasterâ to provide timing sync to the other devices in the group.
Iâve used aggregate devices in the past quite a bit with few problems.
That said, because I use MOTU hardware that include ADAT I/O, I find it easier to run one device as the main interface and feed other device into its ADAT ports. A bit more work to set up, but absolutely rock solid.
InterestingâŚI thought this could only work with wordclock connection but I see there is an option for timing drift correction on non synced devices in the aggregate setup.
I have two of the MOTU 16A AVB interfaces, and I wasnât able to get it working to satisfaction by keeping both interfaces connected to the MBP via Thunderbolt 2 (and using an aggregate device setup), but had no issues keeping just one of them connected to the MBP via Thunderbolt 2, and the second interface connected to the first via the ethernet port via a Cat6e cable. Then when adding my analog gear as an external instrument, it was able to recognize and calculate the latency correctly, depending on which interface my gear was connected to. Running at 96k, my RTL figures were .45ms for gear connected to the first interface (thunderbolt 2) and 1.60ms for the gear connected to the second interface, which is jointed to the first via ethernet. Love the versatility with their new AVB line. Would definitely recommend if you have multiple interfaces you want to use.
Their AVB stuff is high on my list when I update my aging interfacesâŚ
I have to take a deep bow. My ignorance and simplicity overwhelmed my deepest believes. The more uncomplicated at the front, the more complicated at the back. Who would thought that a simple aggregated device conflicts with Cubaseâs ASIO. Steinberg still holds on to their ASIO. Aggegated devices are handled by CoreAudio. Working together? Sorry, was too optimistic to think it would all work flawlessly together. But suggestions still appreciated.