Macro to ORDER unique combinations

Hi

I'm not sure if this is possible, but here goes.

I have three columns of letters

Screenshot (1).jpg

Column A is my list of 5 variables
Columns B and C are my list of unique two digit combinations of column A (done using a macro)

In column D, with the first row (A and B) being combination number 1, I would like to order the list of columns B and C such that the next on the list is the first combination not to contain either of the previous two values, meaning that the next in the list is row 8 (C and D), it should then loop through the list to find the next unassigned row that doesn't contain either C or D which means the next row will be row 4 (A and E) it then goes back through the list to find the next unassigned row not containing the previous two letters which would be row 5 (B and C) and so on and so on.

Any help would be much appreciated!

Many thanks

Martin