Just use a boot loader. I use Grub. Plop is pretty good too.

Some people I know have mounted a single on/off button for powering up a secondary hard drive. This is not much different.

That case system has a relay acting like a DPDT switch, it selects which hard drive gets 5V and 12V. The relay is being controlled by a transistor to avoid huge relay coil current being switched on the front panel, so just a small current to control the transistor is needed on the front panel slide switch. My guess is that there are also two 3.3V regulators, so the thing can be fed only with 5V and 12V Molex connector even if SATA drives need 3.3V for something.

If you have only one hard drive, use a boot loader.

This method might only work on IDE drives, if you use only one IDE drive per cable.

Some BIOSes are able to select the boot drive, or swap hard drives.

If the motherboard supports USB booting, consider putting DOS on a USB stick.
Or you could just use a big DPDT switch to select which hard drive gets current. In any case, don't cut off hard drive power when it is doing something.

So, what I need to make dual boot hdd switches? Can you or someone give me a diagram for DPDT HDD Switch?

I don't dual boot. I have a CF card reader that goes into one of the back slots and I just shut down the machine and swap CF cards 😀

Works very well.