There are already microcontrollers that have both arm + riscv support. This isn't something insane.

RP2350 has separate cores for that. This is one core that speaks both.

To my understanding, the RP2350 has two ARM cores and two RISC-V cores, the two pairs are separate, and only one architecture can be used at a time on the chip.

nope. two "slots", each selectable from {ARM, RISCV}. In each, you pick one. so you can configure it at two arm cores, arm + riscv, or two riscv cores. dual-arch is possible, if you desire it