I have a SSD where Qubes OS is currently installed and I have another SSD which I plan to install Windows 10, but it’s said that it’s not recommended to dual boot Qubes with Windows 10 as it can be dangerous to security and anonymity as per the quote:
If you dual boot Windows and Qubes, it’s possible that a Windows virus could modify your Qubes /boot partition and infect Qubes.
But I want to have Windows 10 installed on the second SSD where only games from Steam will be installed. In this case, would there still be any risk to security or anonymity in Qubes? Judging by the fact that Steam takes the security of their games very seriously and it’s very rare that a game from Steam has any virus that can potentially infect the Qubes SSD.
I don’t think this is something to worry about according to your threat model. A virus that modifies /boot on another disk is pretty rare. It’s still a possibility, but most of them reside and modify their current host OS, that’s it. If you are comfortable with that, you can go ahead.
Do you have a desktop PC or a laptop? It is possible to use dual boot with Qubes OS secure if you will buy sata power switcher and will toggle the used SSD with a button. Windows or any other OS won’t be able to access another disk at all.
Alternative option: change the disk configuration in EFI/BIOS with disabling one SSD and enabling another. It can be used on both PC and laptop. A bit clumsy (not like pressing 1 buttons) but still much better than usual dual boot.