Key space size when either of two public keys are valid for authentication? If for authentication a user can own either A OR B public key instead of just one specific key is that equivalent to halving the key space. i.e. it it theoretically twice as easy to brute force and only equivalent to a 128 bit key?
Yes, having two valid keys in effect halves the size of the keyspace &mdash; from 2256 to 2255 possible keys per each valid one.

Not to 2128 keys, which is what a 128-bit keylength would get you.

Remember that the number of possible keys grows exponentially with the number of bits per key.  (Adding one bit doubles the size of the keyspace, since the added bit has two possible values: 0 or 1.)
A 256-bit keyspace has 2256 possible keys, which is a huge number.  Even if you had literally billions of valid keys instead of just one, it wouldn't make finding one of them by brute force any less impossible.