Open Broadcast Software or OBS is a popular open source program capable of doing many components of video production such as vision mixing and audio mixing.
The software commonly outputs directly over streaming formats such as RTMP and SRT, however, it is possible to have OBS output the mixed, program feed over HDMI so that you can plug the output of your laptop into a LiveU Solo or LiveU Solo PRO and use their bonding capability as your transmission path.
Here are the steps to do so. You can think about setting this up in two parts: routing video and routing audio.
Routing Video to HDMI
This is the easier of the two steps. In OBS, you can right click on either the Preview screen (when not in "Studio" mode), or on the Program preview (in Studio mode), and select Full Screen Projector, and from there, select the specific output from your laptop you want. You will need any HDMI adapter already plugged in, so that when you do right-click, you see all of the display adapters available.
Note, your display settings in MacOS control your options here - you need the HDMI port or adapter to not be set to "mirror" the main display and instead act as an additional display. You can also override the default resolution if desired and set it to another - for instance if you want to output 1080p from OBS and not 4K, when using a LiveU Solo PRO.
Routing Audio to HDMI
This one is a bit trickier. Basically, OBS can route audio to the Monitor device (sort of "Preview" for audio), which in turn is your operation systems sound output device. So the configuration here is three steps.
First, set your OS sound output device to be HDMI or to at least include HDMI among its outputs. This setup is OS specific obviously but one example, in MacOS, open up Control Panel, and go to Sound, then set HDMI as the audio output device:
Next, in OBS, go to the Audio Mixer control panel:
Then, in the Audio Mixer control panel, set which sound sources you want to route to the Monitor output:
In this dropdown box, Output would refer to OBS's streaming output, while Monitor would refer to the HDMI output, since you routed it there in step 1.
Once you have all the audio sources you want set to also go to the Monitor output, they will route to the HDMI output based on your settings in the Audio mixer as normal.
Once you both video and audio routed to your HDMI output, you should see it on the small preview screen on the LiveU Solo unit and be able to stream it using Solo to whatever destination you want.