First off, download Rusted Pack File Manager[github.com] from github.

Go to my mod's directory (\steamapps\workshop\content\1142710\2856024846) and the other mod's directory, and open both with RPFM and select unit_upgrade_group_ui_infos_tables folder, right click and extract it. Do this for both mods.
https://imgur.com/AqVvw10

Extract it to your desktop or anywhere. Then create a new .pack file with rpfm or copy one of the mods, open it and delete everything inside (the entire db folder) and rename the file to whatever you want your mod's name to be. Then in RPFM, right click the name of the .pack file and select add folder and add the previously extracted DB folder(s).
https://imgur.com/lAYJgeo

This should leave you with only one subfolder in DB, that being unit_upgrade_group_ui_infos_tables. Next, open the folder and rename the table to something else so it doesn't conflict with any other mod. Once you've done that, delete all the entries except those relating to the units you want to change. If you're confused about unit IDs, you can search the Total War Warhammer wiki for that unit's info, it usually contains the unit's database ID. Chosen of Khorne[totalwarwarhammer.fandom.com] example:
https://imgur.com/73dg3z7

After that it's just a matter of changing the X and Y co-ordinates of those units to put them in empty spaces. If you want to keep it simple, don't change anything in my mod and only change the positions of the additional units added in the other mod. Once done, save the pack file, and upload it to the workshop if you want, or leave it in your data folder if you just want to keep it for personal use.

Saving your progress often and checking in-game to see if what you've done is working correctly or not is highly recommended. Also, while testing/working on the mod, keep your load order to a minimum. Disable all mods except the ones you need.
Get the Disable Start Up Splash Screens mod, it really helps save a ton of time when you're restarting the game every 5 minutes.
The Console Commands mod also helps when testing new units and upgrades (it lets you spawn units, level them up, unlock all tech etc). You're also gonna need Warband Upgrade Ultimate for it's UI real estate, at least until I push the UI expansion update post Slaanesh DLC.

If you want to have proper links (arrows pointing to and from units correctly), you'll have to do that in unit_upgrade_group_ui_links_tables. Either extract it from the mods or create the table yourself, whichever you find easier. (personally, extracting and editing is much easier than creating from scratch in these cases when you're working on limited units).

Good luck!