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.