After two evenings of struggle, with no resolution in sight I've ripped both fronts to the same point - where shaping for the collar begins - and put them both on one needle...
I'll work the two halves as mirror images of each other. I don't know if I'll ever get it right but I'm confident(ish) I can make it look like it is.
Hopefully I'm at least learning something!
Wish me luck! (If it works you'll be the first to know!)