The following issue concerns only those people who use the tab to switch between the editor windows (IMHO much more intuitive). These people have a smaller problem when it comes to the following in the source segment:
It means to mark the tab and strike the key to copy source to target.
Not really comfortable.
Wouldn't it be a better idea to show it like this and to enter it with the same key stroke as a tag?
Similar things for paragraphs (just to draw the attention on it):
What about this?
Instead of changing the keyboard shortcut to switch focus between editors, you might add the tab character to non-translatables in the form of regular expression:
then you can just use the F4 shortcut, to transfer tabs from the source to the target segment pane.