I will have to strongly disagree that is the easiest. Working with unbound forms requires a lot of programming that a bound form does natively. By far the easiest solution is to bind the form and run an update/append query into the other table.
However, the real question is why? This hints at a non-normalized data structure. Unless you have a strong reason, databases store unique information 1 time. If possible, a redesign of your tables is probably the correct solution.