Use of two private/public key pairs, Sign only and Encrypt only

[Hank Ivy]

What articles exist on having two private/public key pairs, and using one to only sign a document, and the other only for encryption?  The google searchs had articles on one person having a single private/public key pair for both signature, and encryption.  "Advanced" articles did not seem to cover this topic.

Have there been any courts that subpoenaed the private key and its pass phrase of a user?
Would they make a distinction if a user had two pairs, and used them uniquely for signature or encryption?

-- 

Hank Ivy