Joing Tables Using More Than One Field

I have two tables I need to join but there are 2 fields which they
could be joined on.

Using the example Tablles, TableA and TableB below;

TableA
ID1 ID2 Qty
1 Null 4
2 A 5
Null B 6

TableB
ID1 ID2 Qty
Null A 6
3 B 6
4 Null 7
Null C 8

I want to create TableC which will look like this;
ID1 ID2 TableA.Qty Tableb>Qty
1 Null 4 Null
2 A 5 6
3 B 6 6
4 Null Null 7
Null C Null 8


Any ideas?

Regards,
Ciarán