The Underboss is a "line" job, like s supervisor or manager at work. He's responsible for the actions of others.

The Consigliere is a "staff" position. No one works for him, or directly under him. He advises reports directly to the Don.

I think you could consider the Consigliere "higher" in a sense, though, because he could go to an Underboss and relay instructions or orders from the Don.

"Difficult....not impossible"