Thanks a lot guys for your input and information on this. I wanted to crack on with the job so I scavenged the inside piece of the choke splitter. There was good free length on the choke splitter like in your photos Mike.
To get more workable area on the throttle splitter I cut off the thinner threaded end of the splitter housing that the single cable adjuster screws into, which gave me an extra ~1 inch of maneuverability, what a relief (thanks for the idea Bob!). The metal adjuster still screws in on a few threads. Maybe a risk to have this permanently, but feels solid and it's got the lock nut to support it.
After fiddling around with fitting the cables I can see how it (might) be done with the original length of the adjuster and it's probably like what Ed says... you have to push all the cables past the inside piece before seating any of the crimps. If taking apart, start with the handlebar cable, hold the end crimp and push the inside piece away, you might be able to work the cable out. Installing cables, maybe fit the carb cables first but don't seat the crimps then work the handle bar cable through. Is bloody fiddly though, what a laugh.
Cheers.
PS when the new splitter arrives I'll probably re do it, but I'll cover the wire cables when holding them with pliers to avoid wearing them out.