In Sequence, can I add to both ends of a completed sequence to make it 2 sequences?
I've never played this but reading the rules here it seems explicit you can not.


  A connected series of five of the same colored chip either up or down, across or diagonally on the playing surface.


I'm assuming this is the definition of 'Sequence' as its not stated elsewhere in rules.  This however is the key rule with emphasis mine


  If you are playing the game which requires two SEQUENCES to win, you
  may use any one of the spaces from your first SEQUENCE as part of your
  second.


So you can just use ONE of your original Sequence.  If you add onto both ends of a 5 then you will be using more than one. 

I don't see however why you couldn't extend 5 into a line of 9 meaning you have two sequences of 5 as this using just one of the original sequence. But it looks as you you may not extend both ends the way those rules are written.