Choosing their username for them
 

I have two custom fields on my registration form - one is for their firstname and the other for their lastname.

I want their username to always be firstname_lastname (or customfield1_customfield2). Is there a way to make that happen automatically?

You could overwrite the username variable just before it is inserted into the database via plugins.

How would I do that? :)

Create a plugin at register_addmember_process, overwriting whatever the username variable is.

If you're an expert coder you can do it fairly easily. If you're like most of the rest of us, the answer to your original question is 'no'.

In fact, I don't think you can do it anyway in a practical sense. Some things just don't have a software solution. What would stop someone from entering "John <space>" and becoming just "John " on your board?