I run a small company composed of only 2 developers. For one of our clients we are building a very big application, whose development has gone on for 1.5 years.

Now this client has found an important sponsorship, and they are organizing some events related to this project, so we have a deadline in 2 months and we can't miss it.

We are thinking of adding a new developer to the team, and I am wondering what we can do to help his integration.

This is the situation:

• We are approaching the threshhold of Brooks's law, the point when adding new developers will be counter-productive.
• The application is relatively well designed, but the implementation is chaotic in some points (especially older code).
• There are unit tests only for more recent code. When this project started, we didn't have the habit of doing tests.
• Documentation and comments are incomplete.
• The application is both large and complex.
• The client has written down almost every detail about his project, in a very clear and "programmer-friendly" way.

Is it a good idea to add a person now? If so, what can we do in order to help the new developer integrate into the team?