It is an eternal dilemma for small companies like ours – what if the key programmer on a project leaves suddenly! I lived in that dread till I went through it and survived successfully. Here are some notes on that in no particular order.
First and foremost – how so ever small you are, no project which is going to last more than a week should be handled by a single person, this does not mean that you have two people where one is needed – It just means that the other person should be aware of the requirements of the project, should read the the daily reports which the lead programmer sends and keep an up to date CVS check-out of the project.
This leads to the fact that daily reports are a must – these may not be very elaborate just a couple of lines about what was done during the day.
Another thing that you should insist is that all the programmers follow a the same consistent variable, constant and file naming pattern.
The code has to be commented in phpDoc (fill in your fav) style, I personally generate the API docs every 3 to 4 days from the code ensuring that the commenting is coherent enough.
If you can afford keep a junior (fresher) programmer always free as a stand-by
Lastly you as the CTO/Boss should never be so out of touch with any project that you cannot take it over and finish it if needed. It also helps if you have a latest CVS check outs of all the projects on your personal machine safe and secure. Also again remember that while employees will be mostly loyal and honest they almost never understand the intricacies involved in running a business, besides understanding that is not their job anyways.