Is populating one cell based on its unfilled pair possible?

I'm trying to populate one of two cells, whichever is blank, with data based on the second. For example,

.............A................B..............C
...........................Monthly......Annually
1.....Line item 1.......___...........___
2.....Line item 2.......___...........___
3.....Line item 3.......___...........___
....

Where data could be entered into either column B or C, and the other column will perform the appropriate calculation (multiple/divide by 12). And I'm trying to get this to stay in place, so that cell values can be changed repeatedly and the corresponding cell will change automatically.

Is this possible?

Thanks.

Your post does not comply with Rule 1 of our Forum RULES. Your post title should accurately and concisely describe your problem, not your anticipated solution.

Use terms appropriate to a Google search. Poor thread titles, like Please Help, Urgent, Need Help, Formula Problem, Code Problem, and Need Advice will be addressed according to the OP's experience in the forum: If you have less than 10 posts, expect (and respond to) a request to change your thread title. If you have 10 or more posts, expect your post to be locked, so you can start a new thread with an appropriate title.

To change a Title go to your first post, click EDIT then Go Advanced and change your title, if 2 days have passed ask a moderator to do it for you.

(This thread should receive no further responses until this moderation request is fulfilled, as per Forum Rule 7)

You would need code for that I think.

This would go in the worksheet module which you can access by right clicking the sheet tab and selecting View Code.

Private Sub Worksheet_Change(ByVal Target As Range)

    If Not Intersect(Target, Range("B2:C100")) Is Nothing Then
        Application.EnableEvents = False
        Select Case Target.Column
            Case 2
                Target.Offset(, 1).Value = 12 * Target.Value
            Case 3
                Target.Offset(, -1).Value = Target.Value / 12
        End Select
        Application.EnableEvents = True
    End If

End Sub