There is no way to hide a drive from the OS. One option would be to create a truecrypt partition (encrypted "drive") on the SSD drive and simply never decrypt it except when you are booted into the OS you like. Since the decryption key won't ever be used/stored/left in temp files/in memory/etc on the "win8" partition won't be able to decrypt the truecrypt partition.
Baring user error (like accidentally decrypt the secure partition from the "causal OS") there is nothing that can be done. Any malware can see the drive, see the encrypted partition but lacking the decrypt key can't do anything about it.
I figured this might be the only way. Only reason I haven't tried this option yet is I don't have any experience with TrueCrypt or encrypting drives in general. I just don't want to screw anything up and have to format/reinstall/reconfigure everything on the laptop and deal with the potential downtime from work.
Is it as simple as booting in to the encrypted drive prompts me for my key, which then decrypts the drive and loads Win 7 and runs as normal? And when I boot in to the Win 8 drive I just have an encrypted volume visible? That would be good.