Legacy boot should work without intervention out of the box.
I Have installed many times on PC with Win10 on a second hd, installing Grub tu the HD where i have Antergos installed, it allways detect my Windows installation automatic, and create a proper boot entry inside the grub bootloader.
The only thing syou should have in mind is to detect the free space and do not overwrite yout windows partitions.
You need to do a manual partitioning: choose / (root) and swap with the same size as your installed system-Ram. (symple setup) you can also choose /boot with 250MB / (root) with around 35GB and /home with the rest… i would choose also ext4 as filsystem for root home and boot (swap will be swap)
Second is to take care to use grub for install (but as far as i know this is the only option to install bootloader on Bios Systems)
Here you have to choose the option with the wheel: Choose exactly where Antergos should be installed:
On this screen you will only take a look that Use the device below for boot loader installation is choosen and shows your HD:
The Rest is straight ahead and selfexploring:
Some info you will finde on my side here: