Switching on fly between OS1 and OSXL without updating

How do I do this? I've got OSXL installed now on my MPC2500.
I hold tap temp while switching on but I get MPC Update screen.
How is it possible to switch between two OS on fly. I don't want install OS from CF each time I want to switch. It works perfect on MPC1000 this way.

okay.

"Since the program of boot differs from MPC1000, though regrettable, MPC2500 can switched OS and cannot use it.
Since the program of boot cannot be changed, it is impossible."