Mark a cell from one file to another file

I have 2 files with 2 different names. One file, A, uses a popup format to show a list of names. The other one B has same names in a long list down a column. I want to be able to mark, put an x in a cell by the person's name under a specific date, in B file when I choose the same name in A file for the same date. I have no idea how to link these and proceed.

Here are 2 screen shots. One is the popup file and the other is the column file. When I choose the name in popup for a month I want the column file to place a check by that name and the same date.