Imagine a wizard trainee, who is told that the word "expelliarmus" is in fact the "avada kedavra" spell. Would he actually cast "avada kedavra" or "expelliarmus".
In other words can you think of one spell, think another and actually cast the thought spell instead of the spoken one.
This would be a major advantage, if you can do it and I wonder why everyone prefers the unspoken one.
EDIT: to be clear i dont ask for two spells at the same time, but casting another spell than you have spoken.