Graph Theory - Merge two graph variables (with corresponding nodes / edges) of different dimension from Excel ?

I have 2 Excel tables (describing two graph edges), created using the function writetable(G.Edges, myXLSXfile).

The differences between excel files (namely between the two graph variables) are the following:

• one of them could have new columns than the other
• one of them could contain new edges

It means both can also have common rows (=edges) and columns (=edges properties).

Now I want to merge these two tables / graphs in such a way to follow this rules for the resulting graph variable:

• it must contain all columns
• it must contain all edges / nodes
• for common nodes / edges having different properties, just one of them (e.g. always the second input graph/table) has the priority on the other (namely must be choosen).
• for all nodes / edges which didn't have new properties before merging, must be set a default value (e.g. 0)

How to get this result? Thanks in advance.