But you can still run both at once. RP2350 datasheet

From section 3.9 Arm/RISC-V architecture switching:

There are two processor sockets on RP2350, referred to as core 0 and core 1 throughout this document. Each socket can be occupied either by a Cortex-M33 processor (implementing the Armv8-M Main architecture, plus extensions) or by a Hazard3 processor (implementing the RV32IMAC architecture, plus extensions).

From section 3.9.2. Mixed architecture combinations:

The ARCHSEL register has one bit for each processor socket, so it is possible to request mixed combinations of Arm and RISC-V processors: either Arm core 0 and RISC-V core 1, or RISC-V core 0 and Arm core 1.

Practical applications for this are limited, since this requires two separate program images. The two cores interoperate normally, including shared exclusives via the global monitor: a shared variable can be safely, concurrently accessed by an Arm processor performing ldrex, strex instructions and a RISC-V processor performing amoadd.w instructions, for example