Adding a letter.
My fiancee and I were playing last night and she had a vertical word on the board. I added a letter to the end of that word and wanted to also add tiles to the right of the tile I placed, thus trying to get credit for both words. Hi David - The move you describe is perfectly legal. I've included a picture of a similar move played by a visitor who asked much the same question.
She said I can not do this in one turn - I must first add the tile to the end of her previous word, then on my next turn could add the tiles to the right of my single tile.
In that question, TWEAK was on the board and he played FIBS, forming TWEAKS in the process. This is perfectly within the rules.
As long as all the letters you play in a given move form a valid word (e.g. FIBS), it doesn't matter how many other words are formed in the process (e.g. TWEAKS).
I hope that clarifies things for you, but please feel free to use the comments link below if you'd like further explanation.
All the best.