I am not sure if I am too confused to recognize or it is still missing: I would love to arrange the two documents in columns, not above and under. (Sorry, my English…)
Because the screen is wide, not tall.
Sometimes you live for months with something.
Then you post a request.
Then you play around, as hundred times before.
And, for some intelligent reason, you try to use option-click!
Live is strange.
Glad you found it! 2.0 will make it more obvious by changing the icon when you hold down the Option key. (You can also access the split from View > Layout by the way.)
All the best,
Keith