I don’t know these devices but suspect they each appear as Audio Interfaces. Under normal circumstances only one of them would be available at a time.
Cubase wants to see one and only one Audio Interface at any time. It also wants that interface to have an ASIO driver. So if each of those has it’s own ASIO driver you can select and use one or the other but not both.
However some Audio Interfaces do not have an ASIO driver at all (this is common for built in audio and some ‘consumer’ gear). In these situations you can use the Generic ASIO driver that comes with Cubase (or a popular & similar 3rd party driver, ASIO4ALL). Both of these will let Cubase talk to non-ASIO Audio devices. And they can also be configured to to talk to multiple Audio Devices. So you can use these to talk to both devices at the same time. However this will increase your Audio Latency (i.e. the delay between when a sound hits your mic and then come out of your speakers).
Here’s a post you might find useful
Also as a side note, configuring the Audio I/O can get a bit confusing for new folks because everyone’s setup is different and you need to have it configured correctly before anything else works. I’d recommend ignoring for the moment your hardware synths and work on a project or 2 using only the internal Virtual Instruments. And then after you are comfortable with how that works integrate you hardware synth into your rig.