Imagine you have a few more chanels you need for lights and sound effects or whatever and there you would use a Y-cable to have that spare chanel you need.
I would always need to have a working Y-cable as you save quite a bit of work, which might not even be right and be on the safe side with the connections
Join the servos like in the picture. Then, if one travels the wrong way, you can't simply reverse channels. You'll have to rotate the control horn on one servo by 180º and move the horn to a good position. Otherwise you can use the method described by Scott.
If the servo arms are facing from each other that shouldn't happen...
That's my experience anyways.
If you were to do let's say elevator from 2 parts(each controlled by a servo) then you would have to reverse one.