You'll need to remove one of the devices from the pool, there are many ways to do it, easiest is probably to assign both to the same pool and then remove one of them, note that you need to reset pool config first or it be wiped, this should be straight forward but make sure anything important is backed up before starting.
Stop the array, if Docker/VM services are using the cache pool disable them, unassign the cache devices from both pools, start array to make Unraid "forget" current cache config, stop array, reassign both cache devices to the same pool (there can't be an "All existing data on this device will be OVERWRITTEN when array is Started" warning for any cache device), re-enable Docker/VMs if needed, start array, now do a standard pool device removal and when done assign that device to the new pool.