Wife and I have set up our own business although we are both otherwise
employed as well.

Am I right in saying that we'd both have to pay a set rate (£2.40 a
week?) of class 2 NICs as well because of self-employment? Regardless
of how much profit we make or are we exempt if we make less than £5K?

In this case, would it be better for just one of us to declare
ourselves self-employed as a sole trader rather than a partnership so
that only one set of class 2 NICs is paid?