One computer two hds

My FreeBSD laptop also has a docking station with another hd. I know how to install on the other hd but I don't know how to make it work so that I can boot into each os using a bootloader.

Does your laptop supports boot on USB? I am guessing docking station with HDD is like external HDD and you are connecting it to your PC via USB. You can change boot order every time you switch OSes.

Another way may be taking a small partition of your Internal Drive (in which you need to reinstall FreeBSD if there is no free space or nonessential partitions) or if you want Bootloader to work only when the Docker is connected, taking a small partition of it and installing Standalone Bootloader (Search for Super Grub2 Disk). Another option is to install Bootloader into a Flash Stick.

I should warn you I never tried this. I am just guessing. Again I guess, if boot over USB is not possible you have to install Bootloader into a partition of your Internal Drive.
It's not connected via usb. It has the docking station attached. I don't know why but the attached hd is listed as drive one so maybe I can put grub on it. I have used a usb stick for grub on linux so hopefully it will help me figure it out if i need to. Thanks for the tips. I'll give them a try.