Quentin,
I don't think that Teddy would have gone through all that if he had been able to solve the issue with a 2-1 splitter.
Had the same issue on the '71 SF. Tried alloy and plastic body splitter, routed umpty ump different ways under the tank, zip tied, just didn't work for me. Sourced some correct length SF1 throttle cables, installed dual cable throttle and never looked back.
SFC Replica Bitsa came with dual cable throttle, which has been on the bike since '86. Zero problems with carb synch.
Probably be less of an issue if the splitter was located in a rigid location ala Triple frame choke splitter.
Perhaps someone can show a pic without tank of how the bikes left the factory with 2-1 throttle splitter.
Gerald