DataGrid: Editing cells simultaneously

[Faroukh]

I have a datagrid which let’s say has four columns and rows.
The diagonal cells (0,0; 1,1; 2,2; 3,3) are disabled (they have a value of 1).
The values of the other cells must be the same by changing the row and column number. It means the value of the cell 0,1 must be the same as 1,0 or the cell 1,2 the same as 2,1.
That makes a matrix as follows:
1 2 3 4
2 1 5 6
3 5 1 7
4 6 7 1

How can I enforce the schema above, preferably to edit the subject cells simultaneously and to change their background colors.

Thanks,
Faroukh