If you're considering the HiFive Unleashed then presumably size and weight are not an issue.
Also they only ever made about 500 of them and that was 2018 and it would be hard to find one. "Not near end of life". They were EOL five years ago.
They also don't meet your "2+ real-time cores". There is one. I guess you could tell Linux to not use one or more of the four "applications processor" cores. You can ignore the FPU and MMU, but you probably can't disable the caches and branch predictor.
If the SiFive 4+1 setup is acceptable to you then can get those exact cores, at a slightly slower 667 MHz clock speed, in the MicroSemi Polarfire SoC, complete with an FPGA fabric. An example new SBC with this that should be supported for a long time is the BeagleV "Fire" for $150.
The just announced but not yet available Microchip PIC64 GX is also the same thing without the FPGA.
But what about two (or more) Milk-V Duo? A 1 GHz Linux core and 700 MHz real-time core, both 64 bit. $3 with 64 MB RAM, $9.90 with 512 MB. *Extremely* small and light weight. The Linux core has a 128 bit vector processor supported by gcc 14 (asm, inline asm, C intrinsic functions).
Or if you want something better than the Unleashed, there's the Milk-V Mars ($40 with 2 GB RAM) or Mars CM ($34). The five cores are all dual-issue vs single-issue in the Unleashed / PolarFire SoC / PIC64 / Duo and run at 1.5 GHz. The JH7110 in those (and the VisionFive 2 etc) is well documented, with a fully open source 3rd party boot process. You can program the whole thing bare metal if you want.
The only thing I know of with 2+ real-time cores is the SG2380 which has 16 A78-class Linux SiFive OoO P670 cores with 128 bit vector units, plus 8 real-time SiFive X280 cores with 512 bit vector units (the same as NASA has selected for their next gen space probe CPU, replacing PPC750). But that's only just taped out and is probably a Q2 2025 thing.
Well, the Pi Pico 2 has 2 real-time RISC-V cores, but no Linux cores.