If you’ve got a dual-processor motherboard but only one CPU, you may have trouble booting BeOS. If this happens to you, look around in your BIOS for a setting called „MPS for single processor“ and turn it off. This should get you up and running. If you’re running NT on the same machine, NT will no longer boot properly and you’ll have to toggle the setting each time you switch OSs.
Another solution is to enter into the Safe-Mode menu by pressing space during bootup. Here you can also disable multiprocessing. Although you’ll have to do this for every BeOS bootup, this is definitely more convenient if you have to work most of the time with NT.
Probably best to just get a second CPU!