then deselects the first marker, should you deselect zip 1111 or not??
No, zip 1111 and building BBB will be selected finally.
For your 2nd scenario,
zip 1111, building AAA
zip 2222, building BBB
If the data now contains also 1111, BBB and/or 2222, AAA they will also be selected.
It's not possible because if the qlik selection bar already contains 1111, BBB and/or 2222, then at the map side, those markers would already be selected so subsequent selection of them will only cause deselection instead.
Finally yes I implemented with your unique id strategy and got over the hurdle, thanks a lot Erik!