Is it possible to connect to two different self-host gitlabs? If show how do I add another?
What exactly are you trying to accomplish by connecting the two instances?
A High Availability configuration can certainly be done, but it should be noted that this is not a simple or (relatively) easy process, though I am not sure this is what you are trying to accomplish. In any case, I’ve left a couple of links below for more information on HA configurations.
Beyond this, things like Kubernetes Clusters can also be configured, but this may not be what you are looking for either. If you can shed a bit more light on what you would like to do, then I think we can come up with a solution for you.