My take is that it should be based on overall vehicle width. I'd ague that anything over 36" wide is probably unsafe to split with.
Not unreasonable, but 36" excludes a lot of bikes people regularly split with, including R1200RT cop bikes and most adventure bikes. In the latter case at least width is in the bars and you can wiggle them to get through tight spots.
But they're still fat bikes to be fitting through traffic.
It would be more reasonable (for me) to take my widest bike and say much wider is too much

