I think the issue is a matter of bus lanes. not space, but I could be wrong.
If both fit inside the laptop, it may be that they are ontop of the same bus pipe meaning only one can be used at a time.
That said, all the recent reviews point to the fact that ssd tech is not ready to replace spinning drives just yet.
They have much faster seek time, but all other read/write perfomance is overcome by bounds w/ the right laptop drives(not even in raid0). Which means the sum total of perfomance is still way in favor of spinning drives.
But ssd turbo cache has 1 profound advantage, power consumption as Koshin reviewed I think, the cache is very helpful in battery life extension.