If you want a toggle every 256 cycles then the load and reload value is hex equivalent of 65536-256 which is 0xFF00. So I think you should load CMH2 with 0xFF and CML2 with 0x00.

I never have used a 80c592 but I thought the CM2 must be similiar in operation with any other 16-bit compare register and that way I dont think an 8-bit compare is possible as you want to happen.

Raghu