A friend has showed me, that he has encrypted his system with bitlocker first, than again with truecrypt. So he needs input two passwords before the pc boots up. It is interesting,but is it a good idea?
If independent, then chaining tools and encryption mechanisms might add protection if one is compromised. After all, Truecrypt already provides that option in chaining algorithms, and the truecrypt audit is available. But things are rarely independent, and, for example, if the Bitlocker process is subverted or you do not trust MS, then passphrase keystrokes to Truecrypt could be captured, game over. So, I'm not going to say it's a bad idea, but I do think it's some pain for little gain, and risks diversion from the many other threats and vulnerabilities one has - for example, remote attacks which neither truecrypt nor bitlocker does anything about.
I think we have touched on double-encryption before here at Wilders . My view is that there is only a real security benefit if the encryption methods available are inherently weak. Matthew Green has stated that " multiple encryption addresses a problem that mostly doesn’t exist " I respect his vast knowledge and experience in these matters.