I live in a rear-plate-only state. You get one and only one.
If they only require one, why would they pay the expense for providing two? Especially as "two different cars driving around with the same license plate" is one of the things you have plates to prevent. If it were only about taxes, you'd have a prominent tax sticker and be done with it.
My state switched from 1 plate to 2 plates (admittedly a long time ago) and if you had an old single plate there was no requirement to "upgrade" immediately. I assume the same thing has happened elsewhere. I only upgraded when the car failed state inspection because the plate had become less readable.
We get fresh plates with every redesign - you might be able to steal a tax-year sticker from someone's mailbox, but after X years (think it's six) they're going to redesign and an old plate will be obvious. Regardless of how long you've had the old one, when the new design comes up, everyone (except those who pay extra for a "themed" plate, not just a custom set of letters/numbers) gets a new plate.
If they only require one, why would they pay the expense for providing two? Especially as "two different cars driving around with the same license plate" is one of the things you have plates to prevent. If it were only about taxes, you'd have a prominent tax sticker and be done with it.