Gene terminology - is one gene a concrete, single physical sequence?
'just two molecules of one gene'.

They are the same gene. Gene is more of an abstract concept. We say for example that humans have ~ 20,000 protein-coding  genes. It does not mean that we have only 20,000 molecules, one for each gene. We have many more molecules of course. What we mean is that we have 20,000 different entities, each coding for a specific type of protein. If you have two molecules of a sequence in a tube, it becomes questionable whether these sequences can be thought of as genes. I would say that those sequences lose their 'genicity' or their property of being genes because they are not coding for anything in a tube. They lack the cellular machinery that makes them code for something. I would just say you have 2 nucleotide sequences of the gene BCL2 but not that you have 2 copies of the gene BCL2. It's maybe just a personal difference I see though.