Depends what you mean by "hidden".
If you mean to make use of the "hide" bit in the MBR partition table, then you cannot do so with any MS boot manager, because none of them have ever manipulated that bit. (They do respect it, which can make a Windows installation with the bit set, invisible and hence unbootable, but they are unable to dynamically flip it on or off)
If you boot using grub4dos, you can dynamically hide and unhide partitions.
You don't need to use the MBR "hide" bit though. You can just "offline" one OS from the other with a registry hack, or simply remove the disk letter in Disk Management, which will prevent Explorer "seeing" the unlettered partition.
If you want to install W7 twice, and have each with its own boot files (which will be necessary if neither is to see the other), that's easy if they're on separate HDDs. (just disconnect the other as you install each) If you want two copies on one HDD, then you'll need to install one normally, then before booting the DVD to install the next, set the empty second partition "active" in Disk Management so that the installation doesn't use the first OS's BCD as the boot location.