Can anyone recommend an HDMI extender for this?
HDMI 2.1 standard is currently the only one that would have capacity for what you want to do (that is: you want 8k at 60Hz combined, and only v2.1 can do that). But Standard still limits connections to one monitor per cable.
Displayport 1.4a can do exactly what you want (it's called Daisy chain and I use that on my 2x2k monitors via USB-C), so maybe from this stems your confusion.
Bottom line is that with one-cable-per-monitor limit you either have to have multiple HDMI outputs from GPU or use USB-2-HDMI adapters. But for 4k monitor you also need a fast enough USB port, so make sure you use USB3 at least (or Thunderbolt, if you have one). Oh, and make sure that your HDMI is at least 1.4 (but your refresh will be 30Hz max, so for comfy 60Hz you have to have HDMI 2.0 - compatible port in your PC).
Alternatively, if you have a PC that has APU, motherboard is IGPU-compatible - not to confuse with iGPU, which is old word for APU (Asus, for example calls IGPU option the IGFX Multi-Monitor) - and has another HDMI or DP port, you can use that to connect your second monitor to your PC. Upside is that, in best case, you don't have to buy anything extra, downside is that it will put a bit of load on your system RAM and CPU. And again: those ports need to be HDMI 2.0 or DP 1.2 (and DP-2-HDMI cable; fortunately no need for adapter, this is considered downgrade with Displayport).
And there will be a number of other inconveniences, like Windows will not be able to manage color profiles, use night ligth etc., and it may not support moving windowed apps from one display to another (on my PC browsers work well, but games not quite).
