James,
Perhaps a bit more detail would help. Do you want to do a one-time action where textual contents of two fields (like "first_name" and "last_name") are concatenated to populate a third field (like "whole name"), which will then become the field you use instead of those first two? (I assume you want to combine text strings, rather than to add numbers together)
Alternately: do you want a way to have something like a new field that draws dynamically from two other fields (so that you can continue to update "first_name" and see the result in the "whole_name" field (or template)?)
The best solution depends on what you want to accomplish, and what your workflow might be like going forward. (For many purposes, you could just keep the fields distinct at the granular level, and just combine them in any display or output situation that requires them to be seen together.)
-Springer