I have made two versions of my application, a "large" version, and a "mini"/"lite" version.
Should I be signing both of these with the same key? Or should I sign with a unique key for each of them?
What are the repercussions of signing multiple applications with the same key?
Signing the applications with the same key allows them access to each other's data.
Signing is used mainly to identify an application's developer. If anything, you're suppose to sign all applications you make with the same key.
The Android system requires that all installed applications be digitally signed with a certificate whose private key is held by the application's developer. The Android system uses the certificate as a means of identifying the author of an application and establishing trust relationships between applications. The certificate is not used to control which applications the user can install. The certificate does not need to be signed by a certificate authority: it is perfectly allowable, and typical, for Android applications to use self-signed certificates.
Read everything here: http://developer.android.com/guide/publishing/app-signing.html