You could develop software for customer#1 and use the same software for customer#2 with a non-Copyleft license too. The question isn't "does the license enforce that the source must be distributed", it's "will the customer allow me to release the work as open source to begin with". About the only way Copyleft applies here is if you can make a strong case that the work customer#1 wants you to do necessitates (or would be much more expensive without) using a piece of copyleft-licensed source, and therefore the whole work must be covered under a copyleft license. But as a developer with a long work history I've never hit that case, or personally know anyone who has.

Really about the only scenario I can think of where I'd reasonably expect to hit this case is if I were tasked with building the software stack for some kind of appliance that would be greatly simplified by throwing Linux on there. And even then, the custom software running as a process on the box would presumably not have to be Copyleft-licensed, just any changes to the Kernel or any other bundled GPL'd software would have to be made available (if in fact there are any changes), and if GPLv3 is involved then the box would have to have some way for the user to replace the software (but the Linux Kernel is GPLv2 so that may not be an issue).

Barring that scenario, if your idea is "I'll develop something for customer#1 and slap the GPL license on it, then customer#1 has no choice but to release the source and I can reuse it for customer#2", unfortunately that's incorrect. Work done for hire is owned by the employer, not the employee, so you can't slap the GPL license on work you do for customer#1 unless customer#1 authorizes it. And in fact if the entire work is built in-house (rather than pulling in GPL'd libraries), then it doesn't even matter if you claim that it's GPL'd because the customer would be the sole copyright owner and would be able to relicense it. The only reason they couldn't relicense it is if it pulls in third-party GPL'd code or libraries, but if I were customer#1 and I hired a developer who pulled in GPL'd libraries without permission in order to try and force the entire work to be released as open source, you can bet I'd be firing that developer on the spot.

> You could develop software for customer#1 and use the same software for customer#2 with a non-Copyleft license too.

No, I cannot. Work done for hire is owned by the employer, not the employee. But, if employer chose GPL-covered software, e.g. Linux kernel, then he is forced to release code under GPL, so I can develop my code, e.g. kernel module for customer #1 and then use it with customer#2, because GPL protects my freedom. I don't need to reinvent the wheel, like with proprietary code.

If the employer chooses any open-source license whatsoever then you can use the code with the next customer (assuming the next customer is ok with the license). There's nothing special about Copyleft licenses there. Well, except for the fact that a Copyleft license is far more likely to have customer#2 say "no you can't use that, it's GPL, are you crazy?"

Depends on the relationship and contracts in place. If he said employer#1 and employer#2, yes, that's a problem.

"Customer" implies a much looser relationship, but you should still make sure appropriate contracts defining ownership are in place.