Edit: I made a mistake, you can't daisy chain the Cinema Display to the Thunderbolt Display. See @Allan's answer above. The rest still stands.
You need to daisy chain the displays.
Though finding information on this isn't the easiest, Apple did use to put this right on their features page.
Thunderbolt technology in Mac mini is phenomenally fast. It features two 10-Gbps channels for data transfer. That’s up to 12 times faster than FireWire 800 and up to 20 times faster than USB 2. But beyond speed, Thunderbolt gives you unprecedented expansion capabilities. You can daisy-chain as many as six Thunderbolt devices — including two Apple Thunderbolt Displays — to a single port. And since Thunderbolt is based on DisplayPort technology, Mini DisplayPort devices like the Apple LED Cinema Display plug right in, too.
This is probably the easiest and cheapest way to use multiply Thunderbolt displays too.
Why other things won't work
Just to be thorough, I'm going to do a quick run down of why you won't be able to use some other solution besides this daisy chaining.
Solution 1: Just plug one of those cheap Mini DisplayPort to HDMI converters in!
These cheap converters use what's known as DisplayPort++, a method by which DisplayPort actually send out an HDMI signal instead of a DisplayPort one. This means that the adapters can be "passive" and just change pins around, rather than having fancy (and expensive!) conversion circuitry to actually convert the video in real time. This means though that the adapters aren't bidirectional, and you can't convert from HDMI back to Mini DisplayPort.
Solution 2: Use a fancy active adapter
There are active HDMI to DisplayPort adapters that don't use DisplayPort++. However, if the price didn't put you off (they're usually $50-150), the HDMI port on the 2012 iMac only supports up to 1920x1200. You could try using SwitchResX or mac-pixel-clock-patch-V2 to force the resolution up, but it's expensive and prone to failure.
Solution 3: Use an MST hub
Ignoring the fact that you have a Thunderbolt display (which requires Thunderbolt and not Mini DisplayPort), MST isn't supported on macOS. Sure, you could bootcamp all the time, but then what's the point of having a Mac?