You can add multiple remotes to a hub BUT only one can be of the touch screen type like the Ultimate or Elite. This chart tries to explain that but I did not get it the first time through. If you try and add a second touchscreen remote it just replaces the first one though both show in the app and no error is given. (In fact even after I moved the second Ultimate remote back to the second hub it still showed in the desktop app as a third remote till I removed it.) What you need is a special remote, the Logitech Harmony Smart Control Add-On Companion Programmable Remote for Harmony Ultimate Hub. This pretty much works. Since there is no screen and only 3 activity buttons you are limited to 6 activities (short and long press of each button) and just basic controls. At about this point it hit me that the hubs do most of their controlling via WiFi instead of IR so I set up the second hub with the remote it came with and cloned the devices and activities from the first set to it I might be good. Unfortunately controlling devices from 2 hubs can cause state confusion in the hub like I've seen when using the original remotes with the Harmony. Plus I should note here my Onkyo receiver, Roku and Fire TV can be controlled by either hub but my TiVos, Blu-Ray players and HDMI switch can not.
Mapping on Companion remote is not quite right either for my Roamio OTA. For example in TiVo control mode, the MENU button on the Companion remote does not map to the TiVo button like it does on the touchscreen. In fact it, EXIT and DVR are not mapped at all. I mapped MENU to TiVo to match the touchscreen and Exit to Advance since Advance is only on the touchscreen on the Ultimate remote.
[update oddly when I added old Roamio Pro the MENU button was mapped to TiVo on both remotes]
To do this mapping it needs to be done from the phone app.
Select you hub
Select the MENU button in the upper left corner.
Navigate to HARMONY SETUP > ADD/EDIT DEVICES & ACTIVITIES.
Select the Activity you want to customize
Select Customize remote
Select the remote you want to customize.
On the picture select the button you want to remap
Select the device you want it to map to and then the command on the device (you have choice here of mapping short or long press)
Press > at top of page to save (will say syncing)
you can repeat to map other press here if you wish
Press > at top of page to continue which gets you back to button selection
If done mapping select > (which seems to sync again) takes you back to remote select
If done select < to get back to activity select
Select < or > to get back to settings
Press X in upper right corner and it will sync one last time.
Another issue was I have 3 security camera consoles and 3 other PCs I like to be able to swap to via my remotes that are on a HDMI switch leading into the receiver. I was able to setup activities for each. Adding to the TiVo, DVD, Roku and Fire TV though that makes 10 activities which is no problem on the touchscreen remotes but the Companion only lets you map 6. So I though why not use the above to make the 6 PCs 1 activity and map the number buttons to control the switch. This seems to work fine.
One last note. My HDMI switch is a Monoprice 8X1 Enhanced Powered HDMI Switcher w/ Remote and is great. I've had it installed for 4 years now without an issue. For some reason the Harmony hub keeps turning it off even though it is set to never be turned off. I finally went in and remapped the power toggle command to the OSD command on the original remote which actually does nothing with this model.
One other note. The day after I got all the above nice and sorted lightning took out my Onkyo. So I did a patch around till I can get the Onkyo fixed. I thought I'd just stick 8 of the 10 devices on the switch but I was then depressed to find the phone app did not seem to let me do this. Fortunately I remembered seeing you could select more than 3 devices for activity in the desktop app and that indeed did work. I keep finding other bits you can only do in either the phone app or the desktop app. So before giving up you should always try both.