the purpose of encryption is to not store the data unencrypted at all, storing it both ways makes sense during the testing/development phase, but not after you've proved the encryption works.

so my answer is no; the right thing to do is NOT to split the table,and to only store the encrypted values. so i'd have both enc/-un-enc columns in development, in the same table, during testing, but only there.

once i got encryption where i was confident, i would drop the un-encrypted columns and promote to QA/Production.