Well, from a brute force perspective, they have far less options to try now. Effectively you've reduced your key down to a much lower size.

That being said, effectively what you're asking is "What is 25 + 25" and you gave the answer 5. It doesn't really get you any closer than the actual answer 50, it's just wrong.

That being said, I'm not a math or cryptography major, and there might be some crazy hypermath (that is now officially a word) that you can do to reduce the set of possible private keys down to something managable, but it's at least not something obvious.

That being said, this is interesting (for RSA):

"Exposing an RSA Private Key Given a Small Fraction of its Bits"

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CD0QFjAB&url=http%3A%2F%2Ftheory.stanford.edu%2F~gdurf%2Fpubs%2Fexposure.ps&ei=5AlpUa7_LOqligLGg4DoAw&usg=AFQjCNHVIFD6aik02UgaWsmbdiB0EpcgLw&sig2=6OCJzdX3GDcNaSbQ2DBXRg