Let me just suggest this post (Do it because it works!) in About Agile Software Development blog.

A good experience in how a great guy like Paolo Polce can help a team to grow.

Here are the results, as written by the author:

They used to have 2 people on project A and 2 on project B. Now – in a way – they have 4 on A and 4 on B, at least in terms of knowledge.
Velocity was mainted.
We can actually foresee an increase in velocity by the next 10 iterations