What you're trying to do is risky and difficult at best, and disastrous at worst.
Filesystems are designed to occupy an entire partition by themselves. It is "valid" to have a filesystem that is formatted to occupy less than the entire partition, but it would be very much a hack to try and use the unclaimed space for another filesystem.
There are two types of "resize": grow and shrink.
If your initial state is that one filesystem occupies the entirety of a partition, and you grow the partition (make it larger), that basically means you tweaked the partition table (just a bunch of numbers in a small area of the hard drive, the format depends on your partition type, e.g. MBR or GPT) so that there will be more blocks allocated to the partition than the filesystem is using. The filesystem will not use the additional blocks at the end of the partition, because the filesystem still thinks it's operating on a smaller partition than it actually has. A "smart" filesystem COULD easily detect and start using the remaining space, but I don't know if any actually do that. Either way, you'll probably get at least warnings when mounting your drive.
If you try to shrink the partition (make it smaller) without also shrinking the filesystem, you WILL lose data: when the filesystem tries to write to blocks that are beyond the end of the partition, the device node will throw an exception for trying to write past the boundary of the partition, and the filesystem will throw an I/O error up to userspace. Any data you previously had in the re-partitioned space will be lost.
Either way, you're shooting yourself in the foot. I have no idea how you would even begin to tell a filesystem to mount a filesystem starting at a certain block within a partition. A command like mount /dev/sda1 implies that the mount starts at the very beginning of the partition. Yes, you would be slightly obscuring the location of any data in the "hidden partition" if you were able to create a filesystem in there; but anyone savvy enough to want to poke around on your hard drive is going to very easily find it, so don't think you're gaining any measure of security.
If you are worried about other users of the computer or unauthorized persons with physical access to the computer, you should use full disk encryption based on strong cryptography. Merely hiding a partition isn't going to confuse most physical-access users for very long if they are looking for interesting things on your disk.
Check out fdisk or cfdisk if you still want to do this, but you're voiding every warranty in the book (and then some) by doing this...
