Discovered by accident.
You're in layout mode. You define a cond format for a field, F1. Now you want to apply it to another field, F2.
1) You select both F1 and F2, invoke Format > Conditional... and you see nothing, an empty dialog.
2) You select F1, Arrange > Bring to Front.
you do 1) again.
TaDaa, now you see F1's cond formatting and if you press OK it will extend to F2.