The super easy way would be: While installing Win10 on new SSD, disconnect all SSDs and HDDs. After finishing the installation, connect them back. Then set the default boot drive to the SSD with the OS you'd use the most (for example Win7) on BIOS. Now, if you want to boot into Win7 just let the PC boot normally. And when you want Win10, during boot press F8 (for Asus mobo) and select the Win10 SSD.
The moderate and my preferred way would be: Install Win10 on the new SSD while keeping the old SSD connected, but remove all other HDDs. After installation completely finishes, check which OS boots up by default (re-check by restarting from both OSes if they both are available at startup) a few times. After being sure, install EasyBCD in that OS and tinker around. I'm sure you'd need to poke around a bit to get everything going, and I think even I wouldn't be able to make it work in first try.
The super hard way: Edit boot records, install grub, set up nested bootloaders etc. etc. etc. You might run out of hair to tear from your head out of frustration while figuring out everything. But you would be in complete control of every step of the booting process of your machine and can do limitless things with it.
Ultra super hard way: Wipe both SSDs, install VMWare ESXi or some other hypervisor that allow gpu pass through. Install both OS on virtual machines inside that. You'd be able to run bot OS simultaneously if you want.
Now honestly, why would you need an entire new SSD for Win10 if you are not going to use it often? Just partition the existing one if you have enough free space. You can make a 20GB partition to install the windows, and then install the game to some other partition, probably the one you'd install games while on Win7. If you want to install windows store games then use
this guide.