If you use the "full" editor (i.e. the one will all the buttons), quote a message and you obtain something like:
[quote]
this is a quoted message
with more than one line
[/quote]
If you put the cursor in the empty line:
[quote]
this is a quoted message
<---- CURSOR HERE
with more than one line
[/quote]
and use ctrl+enter from the keyboard, it splits the quoted text to:
[quote]
this is a quoted message[/quote]<--- the cursor is here
[quote]
with more than one line
[/quote]
You have now split the quote without having to copy the quote tag or write a new one, etc. to end up with:
[quote]
this is a quoted message[/quote]
This is my reply to the first line
[quote]
with more than one line
[/quote]
And here I answer your second line.