On iTunes I see that each translation requires a different amount of storage space. For example, the NKJV needs 10.8 MB and the NASS needs 17.8 MB. If I want to download both of these versions, are there any features that overlap and get eliminated, so that in total I don't need a total of 28.6 MB of free space to use both translations together? If so, does anybody know about how much space gets freed up by this?

The program itself takes a few megs of space. Everything else is taken up by the version(s) you download. But we are talking about a TINY amount of space, here, relative to the amount of space available on an iPhone or iPod Touch.

For example, the smallest device available has 8GB of space (about 8000mb). If you had the 4.0 program, plus the KJV and the NASS (with it's notes and strongs dictionary installed) and the NKJV (with it's notes) installed, you'd need about 80mb of space. That's about 1/100th of the space you have available.