Same issue here with R-Go split
Interestingly I found one configuration that works, but horribly breaks other things:

first, have a look with xinput

$ xinput 
⎡ Virtual core pointer                    	id=2	[master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer              	id=4	[slave  pointer  (2)]
...
⎜   ↳ Hantick USB Keyboard Consumer Control   	id=20	[slave  pointer  (2)]
⎜   ↳ HID 0911:2188 Consumer Control          	id=22	[slave  pointer  (2)]
⎜   ↳ HID 0911:2188 Mouse                     	id=24	[slave  pointer  (2)]
⎜   ↳ Hantick USB Keyboard Mouse              	id=25	[slave  pointer  (2)]
⎣ Virtual core keyboard                   	id=3	[master keyboard (2)]
    ↳ Virtual core XTEST keyboard             	id=5	[slave  keyboard (3)]
...
    ↳ Hantick USB Keyboard                    	id=16	[slave  keyboard (3)]
    ↳ HID 0911:2188 System Control            	id=17	[slave  keyboard (3)]
    ↳ HID 0911:2188                           	id=18	[slave  keyboard (3)]
    ↳ Hantick USB Keyboard System Control     	id=19	[slave  keyboard (3)]
    ↳ Hantick USB Keyboard Consumer Control   	id=21	[slave  keyboard (3)]
    ↳ HID 0911:2188 Consumer Control          	id=23	[slave  keyboard (3)]
    ↳ Hantick USB Keyboard Wireless Radio Control	id=26	[slave  keyboard (3)]


The Hantick devices as well as the 0911:2188 devices are the R-Go Split (why so many?)
interestingly there is a wireless radio control???
Anyway, let's create a new group

$ xinput create-master rgosplit 0
$ xinput
⎡ Virtual core pointer                    	id=2	[master pointer  (3)]
...
⎡ rgosplit pointer                        	id=27	[master pointer  (28)]
⎜   ↳ rgosplit XTEST pointer                  	id=29	[slave  pointer  (27)]
⎣ rgosplit keyboard                       	id=28	[master keyboard (27)]
    ↳ rgosplit XTEST keyboard                 	id=30	[slave  keyboard (28)]

and now add those 0911:2188 keyboard devices to it (note that you need to replace the numbers in the commands)

$ xinput reattach 17 28
$ xinput reattach 18 28
$ xinput reattach 23 28
$ xinput
⎡ Virtual core pointer                    	id=2	[master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer              	id=4	[slave  pointer  (2)]
...
⎜   ↳ Hantick USB Keyboard Consumer Control   	id=20	[slave  pointer  (2)]
⎜   ↳ HID 0911:2188 Consumer Control          	id=22	[slave  pointer  (2)]
⎜   ↳ HID 0911:2188 Mouse                     	id=24	[slave  pointer  (2)]
⎜   ↳ Hantick USB Keyboard Mouse              	id=25	[slave  pointer  (2)]
⎣ Virtual core keyboard                   	id=3	[master keyboard (2)]
    ↳ Virtual core XTEST keyboard             	id=5	[slave  keyboard (3)]
...
    ↳ Hantick USB Keyboard                    	id=16	[slave  keyboard (3)]
    ↳ Hantick USB Keyboard System Control     	id=19	[slave  keyboard (3)]
    ↳ Hantick USB Keyboard Consumer Control   	id=21	[slave  keyboard (3)]
    ↳ Hantick USB Keyboard Wireless Radio Control	id=26	[slave  keyboard (3)]
⎡ rgosplit pointer                        	id=27	[master pointer  (28)]
⎜   ↳ rgosplit XTEST pointer                  	id=29	[slave  pointer  (27)]
⎣ rgosplit keyboard                       	id=28	[master keyboard (27)]
    ↳ HID 0911:2188 System Control            	id=17	[slave  keyboard (28)]
    ↳ HID 0911:2188                           	id=18	[slave  keyboard (28)]
    ↳ HID 0911:2188 Consumer Control          	id=23	[slave  keyboard (28)]
    ↳ rgosplit XTEST keyboard                 	id=30	[slave  keyboard (28)]

Now I can type with both keyboard halfs simultaneously in a gnome-terminal!
But when I switch to other applications, e.g. firefox, only one half can type...
Maybe someone can build up on this?